Carter BBD Manual
 BBD-144”
SERVICE MANUAL
CARTER CARBURETOR
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TABLE OF CONTENTS
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High Speed CirCUitS ae SESSIONS
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Manual Altitude COMpensatOr ..….….….…...….........….……eerenenenneneneneene rase mana e ana u 0e
Pump Circuit .
Choke Circuit .
Modulated Choke Pull- Off. EEE ——
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Tamperproof Choke ..................e.eresenerercorrenerr DIA
Fast Idle And Unloader .........................
E.G.R. Control .
Wide Open Throttle pute Valve A CN A AEA VV ERE
Throttle Positioner [email protected] …...….....…..........srsereraseenennenencrnnen ren nes a en nn annees
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Dash Pot .
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Transducer And Ground Switch .
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Stepper Motor ..............
Checking Stepper Motor Ct AP PR ES
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Checking Pulse Solenoid ......—...=.=w=.=.....1060siiiica DE a ns ne
Exploded View — Air Bled Design ...........................e.eeereeee..
Exploded View — Solid Fuel Design .................=.=........iímerer DI DD ER EL
Adjustments — Air Bled Design .....................e—ee.....=ereeeccentene ee Te eE
Adjustments — Solid Fuel Design ...........................m000000 DA.
Copyright © 1982 Carter Automotive Division, A.C.F. industries, Inc. St. Louis, Missouri
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DESCRIPTION _
The model BBD is a BB design. dual carburetor with low
overall height, accessible adjustments and removable
3. High speed circuit
4. Pump circuit
5. Choke circuit
All BBD carburetors prior to 1974 are of the air bled
design incorporating downhill nozzles. 1974 and later
subassemblies.
It has been supplied in both 1144 and 114 inch SAE
flange sizes meet emission standards while still maintaining maximum
Five conventional circuits are used. They are: response and driveability. For increased life and smooth
1. Float circuit operation the solid fuel design uses a teflon-coated
2. Low speed circuit
models are of solid fuel design with uphill nozzles. The
solid fuel design prevides more precise fuel metering to
throttle shaft.
O
INLET
FLOAT
FULCRUM
PIN
FLOAT CIRCUIT
All fuel enters through the fuel inlet fitting in the bowl.
The fuel inlet needle seats directly in this brass fitting and
is controlled by the twin or dual floats which are hinged by
a float fulcrum pin. The fulcrum pin is held in position by
the “horseshoe” retainer. The twin floats follow the con-
tours of the fuel bowl and are designed to provide a stable
fuel supply under all conditions. Only a minimum of fuel is
maintained in the carburetor, preventing excessive fuel
evaporization. This tends to improve warm engine starts.
The float circuit must constantly maintain the specified
fuel level as the other circuits are calibrated to deliver the
proper mixture only when the fuel is at this specified level.
When the fuel level in the bowl drops, the float also drops
permitting additional fuel to flow past the inlet needle into
the bowl.
The bowl is vented to the inside of the air horn. The bowl
vent is calibrated to provide proper air pressure above the
fuel at all times. To assure a positive seal, always use a
new bowl cover gasket when reassembling. An air leak at
this point can result in a mileage complaint.
Float Adjustment
Hemove float to adjust. NOTE: To obtain the proper
alignment it may be necessary to bend float lip at either or
both arrows “A” and "B. CAUTION: Never allow needle to
be pressed into seat when making the adjustment.
FLOAT LIP
After the float adjustment has been made and set to the
manufacturer's specifications, the float lip must be in the
vertical position with the needle lightly seated.
Venting the System
The BBD carburetor also uses an outside bowl vent that
opens at the closed throttle position. When the engine is
turned off, underhood temperatures increase causing
vapors to rise from the fuel in the bowl. The outside vent
improves starting characteristics as it prevents vapors
from entering the bore of the carburetor by way of the
inside vent.
Bowl vapor vent adjustment must be to specifications. If
valve does not open to specifications with throttle valves
seated, bowl vapors cannot escape freely and this may
cause “hard-hot-starting.” If it opens too far, or hangs
open, it will allow an external vent to the bowl, resulting in
poor mileage.
Emission Laws effective in 1971 required all outside
vents to be routed to a canister to prevent evaporative
emissions.
PUMP
OPERATED
TWO WAY BOWL VENT
To meet evaporative emission regulations, late model
BBD's use a solenoid controlled two way bowl vent.
When the ignition switch is in the off position. the spring
loaded diaphragm forces the puck valve to its upper
position, thus closing the inside carburetor vent and open-
ing the canister vent. When the ignition switch is tumed
on, the solenoid is energized moving the puck to its
downward position, thus closing the canister vent and
opening the inside carburetor vent.
If the solenoid should fail, venting to the carburetor fuel
bowl would be by way of the canister. This change in bowl
pressure would effect driveability. An increase in bowl
pressure causes a rich condition. lowering bowl pressure
results in a lean condition.
q mm
MAIN USTER
METERING SCREWS (2)
| RODS (2)
À
SPEED
| JETS
TRANSFER
| MAIN Sor
METERING
er ий
MIXTURE 5 L_ ‘ PORT
ADJUSTMENT „г OSI
SCREW
LOW SPEED CIRCUIT
Fuel for idle and early part throttle operation is metered
through the low speed circuit.
Fuel enters the idle and high speed wells through the
main metering jets. The low speed jets measure the
amount of fuel for idle and early part throttle operation.
The by-pass, idle air bleeds and economizers located in
the venturi attaching screws, are carefully calibrated and
serve to break up the liquid fuel and mix it with air as it
moves through the passage to the idle ports and idle
adjustment screw ports. Turning the idle adjustment
screws toward their seats reduces the quantity of fuel
mixture supplied by the idle circuit.
The idle ports are slot shaped. As the throttle valves are
opened, more of the idle ports are uncovered allowing a
greater quantity of fuel and air mixture to enter the car- |
buretor bores. |
The by-pass, idle air bleeds, economizers, low speed
jets, idle ports, idle adjustment screw ports, as well as the
bores of the carburetor flange, must be clean and free of
dirt and carbon. Obstructions will cause poor low speed |
engine operation.
Idle Adjusting Screw
The idle bleed is into the bore of the carburetor on the
atmospheric side of the closed throttle valve. The amount |
of bleed varies with throttle position.
Idle adjusting screws are used for trimming the idle
mixture to individual engine requirements for satisfactory |
idle. |
Emission Laws require use of idle adjusting screws
with limited adjustability. This allows for proper idle ad-
justment while assuring the emission limits will not be
exceeded.
One design uses an allen screw as a stop as it makes
contact with the shoulder on the recessed portion of the
idle adjusting screw.
On flow test, the idle adjusting screw is tumed in the
counterclockwise direction to the mean rich limit. The
allen screw is then tumed in against the recessed shoul-
der of the idle adjusting screw. The allen screw hole is then
filled with a lead plug.
Another version uses an idle adjusting screw which is
completely recessed in the flange of the carburetor.
After final adjustment, it is sealed with a lead plug.
The upper adjusting screw is an air adjustment screw
and adjusts the mixtures for both bores.
This air adjustment screw has left-handed threads.
Turning the adjusting screw counterclockwise moves the
screw inward to richen the air-fuel mixture.
/
|}
ph
Adjustable Off-Idle Air Bleed
Some older BBD models use an “adjustable off-idle air
bleed” which is adjusted during flow test. This adjustment
should never be changed as it cannot be adjusted in the
field.
The purpose of the “adjustable off-idle air bleed" is
greater control of the air-fuel ratio at flow rates above curb
idle, resulting in substantial reduction in hydrocarbons.
The circuit consists of an adjustable spring loaded ball
check valve located in an air passage to the low speed
circuit.
Closer calibration can be attained by being able to
adjust the idle fuel mixture at two different points in the
air-fuel ratio curve.
The air bleed valve is set to open at an idle port vacuum
of 7 to 12 inches of water which is slightly above the three
to four inches at curb idle.
When the throttle is opened slightly, the lower pressure
at the idle port opens the air bleed valve to control the
air-fuel ratio. |
If the rate of acceleration reaches a certain maximum,
the vacuum at the idle port will drop below the 7 to 12
inches of vacuum allowing the air bleed valve to seat. This
enrichment of the air-fuel mixture is desirable for a high
rate of acceleration.
As the bleed port is below the closed position of the
choke valve, air will not enter the air bleed valve until the
choke valve is partly open, thus making the automatic air
bleed inoperative during the early stages of engine warm-
up.
The air bleed valve will open on deceleration from high
speed to prevent rich mixtures.
The “adjustable off-idle air bleed" is also used on some
AVS models.
Idle Limiter Caps
All late model carburetors use idle limiter caps to pre-
vent over-rich idle adjustments.
IDLE RESTRICTOR
TRANSFER SLOT
IDLE MIXTURE
IADJUSTMENT SCREW
IDLE PORT
Idle Restrictors
In addition to limiter caps, some late models use idle
restrictors located in the throttle body. The purpose of the
restrictor is to limit the maximum air-fuel enrichment avail-
able at idle.
Tamperproof Mixture Screws
Some 1979 and later model carburetors will use the
hidden “tamperproof idle mixture screws.” These screws
are adjusted and sealed at the factory. Adjustment of the
sealed idle mixture screws should be performed only
when the carburetor will not meet specifications or when a
major carburetor overhaul is necessary.
Idle Enrichment System
Some models use an IES, “Idle Enrichment System,” to
improve cold engine performance at initial starting of the
engine. Along with this carburetor circuit or system, an
electronic timer and vacuum solenoid valve are also used.
The timer energizes the solenoid valve during starting and
for 35 seconds after start. When the solenoid is energized
it cuts off the EGR system which eliminates any exhaust
gas recirculation from taking place. Secondly, it applies
manifold vacuum to the idle enrichment diaphragm when
the engine coolant is below a predetermined temperature.
The manifold vacuum overcomes the spring tension and
pulls the diaphragm away from the seat and valve,
thereby allowing the valve to seat closing off the air
passage. Cutting off the air supply enriches the idle mix-
ture which allows more fuel to be delivered during starting
Idle By-Pass Assist
|
|
|
AIR INLET
PASSAGE
IDLE
| CAVITY
and for 35 seconds after start. After the 35 second delay
period, the electronic timer de-energizes the solenoid
valve which allows the EGR system to function and also
| cuts off the manifold vacuum to the idle enrichment dia-
| phragm. The spring then pushes the diaphragm against
the valve and seat assembly causing the valve to unseat.
This in turn allows air to flow through the system and
| normal air-fuel ratio to be delivered to the cylinders. The
purpose and results are improved hot engine starting by
delaying the EGR for 35 seconds and improved cold
engine performance after starting by initiating idle en-
richment for 35 seconds.
Some BBD units incorporate an idle by-pass assist.
| This passage goes through the main body, the body
gasket, through a passage in the throttle body and enters
below the throttle valve. This extra air through the by-pass
allows the throttle valve to close a little more for a given
idle RPM. This reduces the CFM air flow over the nozzle
tips and prevents the possibility of taking fuel from the
nozzles during fast idle operation. It also causes a tur-
bulence below the throttle valves to aid air-fuel mixture
and distribution.
idle Solenoid
Many carburetor models use an idle solenoid to prevent
“dieseling” or “after run.”
Many things that have been done to lower emissions
have enhanced the possibility of dieseling. Higher idle
speeds, leaner air-fuel mixtures, retarded ignition timing,
higher operating temperature, all contribute to dieseling.
When the ignition is turned on, the solenoid is energized
moving the plunger outward. The idle RPM is adjusted at
the solenoid. When the ignition is turned off the solenoid is
de-energized, the plunger moves inward allowing the
throttle valves to close enough to virtually shut off the air
supply, causing the engine to stop running immediately.
Some units have a second adjustment to prevent the
“throttle valves from closing too tightly.
Air Conditioner Solenoid
The air conditioner solenoid is used on many appli-
cations to maintain idle RPM.
The extra load on the engine when the air conditioner is
tumed on causes a drop in idle RPM.
The solenoid is energized moving the solenoid plunger
outward. This outward movement opens the throttle
valves (as specified) to maintain idle RPM.
SIS SOLENOID
Some later models use a S.1.S. (solenoid idle stop)
solenoid. When the air conditioning, rear window de-
fogger or any accessory with a heavy load is tumed on,
the S.I.S. solenoid is energized and the plunger moves
outward to open the throttle valves slightly.
The adjustment of the S.1.S. solenoid is on its inward
travel rather than the conventional outward travel. Two
adjustments are required and must be made in proper
sequence, as specified on the solenoid decal.
When the accessory is tumed off, a timer gives a two
second delay in de-energizing the solenoid to prevent
engine die out.
SOL-VAC
The sol-vac is also used on many applications. The
electrical solenoid is energized when the air conditioning
is on, when the hedge hog is in operation, rear window
defroster or any heavy electrical load.
The vacuum portion is activated anytime the air tem-
perature in the air cleaner is below 55 degrees, or anytime
the idle drops to 450 R.P.M. At 450 R.P.M. the vacuum
section is activated and opens the throttle valves to speci-
fications which is above normal idle. A time delay is used
“| SOLID FUEL CIRCUIT
to retum the throttle valve to normal idle. If idle drops to
450 H.P.M. the second time, the vacuum unit is again
activated, however the time delay is not in operation. A
return to idle then requires increasing engine speed to
1150 R.P.M.
The hedge hog replaces the heat riser. It is a finned type
heater element located in the manifold just below the |
carburetor. It is controlled by a wax pellet type tempera- |
ture switch located in the engine block. The hedge hog is |
on any time the water temperature is below 160 degrees. |
Three adjustments are required and must be made i “a |
the proper sequence.
AIR BLEED CIRCUIT
VACUUMT
PISTON |
} STEP-UP
y e
| VACUUM _
PASSAGE |
METERING — Y
ROD (2)
HIGH SPEED CIRCUIT
Fuel for part throttle and full throttle operation is sup-
plied through the high speed circuit.
The air bled circuit used prior to 1974 has an emulsion
tube or vent tube that extends downward into the high
speed well. This tube mixes air with the fuel before it
leaves the high speed well. The air bled design always
uses “down hill” nozzles. The air bleed in the high speed
circuit also serve asan anti-percolator passage.
STEP-UP, — —. |
| PISTON EST r /
METERING
RODS (2)
NOZZLE
VENTURI
THROTTLE VALVE —
The solid fuel design, 1974 and later, takes solid fuel
from the dae Miu agan Bt Eds e Leste Ade
the top through the extended vent tubes located in the
cluster, closer to the tip of the nozzle. The solid fuel design
always uses “uphill” nozzles and gives a closer calibration
to meet the emission standards and also serves as an
anti-percolator passage.
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Diminishing Well Bleeds
Some solid fuel models use diminishing well bleeds.
This bleed is subjected to venturi pressure changes that
follow engine load conditions. They serve as self adjusting
air bleeds and at or near wide open throttle, could deliver
fuel.
The two center holes are the pump discharge windows
and also the air bleed to prevent pump pull over.
Metering Rod
The position of the metering rod in the main metering jet
controls the amount of fuel admitted to the discharge
nozzle.
The metering rod has varying step diameters which
controls the effective size of the main metering jet in which
it operates.
Function of the Metering Rod
The two metering rods are yoked to a single step-up
piston assembly which rides in a cylinder in the bowl
casting. The jets which work with the metering rods are
located in the fuel bowl. Note the solid fuel jets are different
than those used in the air bled system.
At part throttle and cruising speeds, increased air flow
through the venturi creates a low pressure area in the
venturi. Since the air above the fuel level in the bowl is
near atmospheric pressure, fuel flows to the lower pres-
sure area created by the venturi. The fuel flow moves
through the main jets to the main nozzle as it picks up air
from the air bleeds.
During heavy road load or high speed operation, the
air-fuel mixture must be enriched to provide increased
engine power. Power enrichment is accomplished by
movement of the metering rods which are attached to a
single yoke and piston actuated by the manifold vacuum.
The metering rod piston rides on a calibrated spring which
attempts to keep the piston at the top of the cylinder. At
idle, part throttle or cruise conditions when manifold vac-
uum is high, the piston is drawn down into the vacuum
cylinder, compressing the vacuum piston spring. The
larger diameter of the metering rods will be positioned in
the main jets allowing a calibrated amount of fuel flow to
the nozzle. Under any operating condition where the
tension of the vacuum piston spring overcomes the pull of
vacuum under the piston, the metering rods will move
upward so the smaller diameter step is in the jet. This
permits the necessary additional fuel flow to be metered
through the jets.
The metering rods in the air bled units are vacuum
controlled, no adjustment required.
The metering rods in the solid fuel unit are both me-
chanically and vacuum operated and must be adjusted.
The lifter tab lifts the metering rods mechanically and also
limits the amount of lift from the vacuum piston
Vacuum Step-Up Piston
Hex-Head Screw
Never attempt to change the factory setting of the
vacuum step-up piston hex-head screw as it will seriously
affect performance. This adjustment is made during flow
testing and cannot be duplicated in the field.
An air leak past the gaskets sealing the ventun cluster.
venturi cover and tube assembly or the ventun cluster
screws will affect both low speed and high speed per-
formance. To assure a positive seal always use new
gaskets and be sure venturi cluster screws are tightened
securely.
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Manual Altitude Compensator
To meet emission standards at 4,000 feet above sea
level, some BBD carburetors use a manual alcomp or
‘altitude compensator.” It consists of a spring-loaded
adjustable cap added to the ventun cluster. During pre-
delivery of the vehicle for altitude use, the adjusting screw
is turned in the counterclockwise direction. The spring
forces the cap upwards uncovering the auxiliary air bleeds
to the low speed circuit. In addition to the auxiliary air
bleeds, there is an oversized air bleed drilled into the lower |
section of the venturi cluster assembly and with the cap in
its upward position, air is bled into both the low speed and
high speed circuits to lean out to the altitude calibration
required. There is no adjustment. The cap merely opens
or closes these additional air bleeds.
PUMP ARM
ACCELERATOR
PUMP
PUMP
| DRIVE
| SPRING
dE LINK
(> PUMP
: DISCHARGE
DISCHARGE
CHECK BALL
PUMP CIRCUIT
The accelerating pump circuit provides a measured
amount of fuel which is necessary to insure smooth
engine operation for acceleration.
When the throttle is closed, the pump plunger moves
upward in its cylinder and fuel is drawn into the cylinder
through the intake check. The discharge check is seated
at this time to prevent air being drawn into the cylinder.
When the throttle is opened, the pump plunger moves
downward forcing fuel out through the discharge check
and out of the pump jets. As the plunger moves down-
ward, the intake check is closed preventing fuel from
being forced back into the bowl.
The discharge check ball is 5/32". The intake check is
316.
The calibration of the pump spring and the size of the
jets provide a pump discharge of the desired duration.
The accelerating pump stroke adjustment provides a
means to assure the proper pump discharge volume.
High air velocity passing over the pump jets causes a
low pressure area. An air bleed located between the
discharge windows and the pump jets prevent pump
pull-over.
BOWL VENT VALVE
CLIP POSITIONS
PLUNGER
SPRING SEAT
SPRING
BOWL VENT VALVE
OPERATING CLIP
NK688
Pump Plungers
Solid Cup Plunger
— Sliding Cup Plunger
After engine shutdown heat can cause vapors to accu- |
mulate within the pump cylinder. The BBD pump plunger
is designed to relieve this vapor pressure and to maintain
solid fuel in the pump cylinder at all times.
The air bled unit uses a “solid pump plunger” with a |
vapor vent passage through the plunger. The solid fuel
unit takes advantage of a sliding cup that gives no bleed
during acceleration. When at rest, it serves as a release
for any vapor pressure in the pump cyliner.
Some 1978 models do not use the intake pump circuit
or intake check ball. These models take advantage of the
sliding “pump plunger cup” and fill from the slots at the top
of the pump cylinder.
u J
CARBURETOR
TO INTAKE
MANIFOLD
HOT AIR
FROM EXHAUST
MANIFOLD STOVE
CHOKE CIRCUIT
The automatic choke circuit provides a correct mixture
necessary for quick cold engine starting and warm-up.
Some BBD carburetors use an integral choke, while
others use the cross-over (Remote mounted type).
When the engine is cold, tension of the thermostatic coil
holds the choke valve closed. When the engine is started,
air velocity against the offset choke valve causes the valve
to open slightly against the thermostatic coil tension. The
intake manifold vacuum applied to the choke piston also
tends to pull the choke valve open. The choke valve
assumes a position where tension of the thermostatic coil
is balanced by the pull of vacuum on the piston and force
of air velocity on the offset valve.
When the engine starts, slots located in the sides of the
choke piston cylinder are uncovered, allowing the intake
manifold vacuum to draw warm air heated by the exhaust
manifold through the choke housing. The flow of warm air
in tum heats the thermostatic coil and causes it to lose
some of its tension. The thermostatic coil loses its tension
gradually until the choke valve reaches full open position.
It the engine is accelerated during the warm-up period,
the corresponding drop in the manifold vacuum allows the
thermostatic coil to slightly close the choke which provides
a richer mixture.
CHOKE DIAPHRAGM
CONNECTOR LINK
CHOKE
CHOKE = CONNECTOR
CHOKE ““ CHOKE
COIL HOUSING
| REMOTE CHOKE
7 WITH ELECTRIC |
ASSIST FEATURE|
When the crass-over type choke is used, the carburetor
mounting gasket is most important. If it is not to specified
thickness, it upsets choke calibration due to the length of
the choke rod. Most cross over chokes are non ad-
justable.
Choke Pull-Off
On many BBD units, the choke piston is replaced by a
device called a choke pull-off. The choke pull-off is a |
diaphragm-type unit that performs the same function as
the choke piston. It opens the choke valve to a pre-
determined opening when the engine starts. The amount
of pull-off is adjusted by shortening or lengthening the
choke pull-off rod.
Modulated Choke Pull-Off
Many units use a modulated-type choke pull-off. In
addition to the regular diaphragm spring, the diaphragm
shaft incorporates a spring within the shaft to provide
better warm-up fuel economy by allowing the amount of
choke valve opening to vary with the torque of the choke
coil spring. This spring-loaded diaphragm shaft merely
allows a temporary tighter closed choke valve during the
very early stage of the warm-up period.
CONTROL
SWITCH —
(DUAL STAGE)
— CHOKE
TU TRS
Electric Assist Choke
Electric assist chokes are. used to help reduce HC and
CO emissions during starting and warm-up. It gives a
closer choke calibration during the warm-up period, This
device consists of a heating element located in the choke
cap on integral chokes, or is built into the remote choke
assembly on manifold mounted chokes. A wire from the
heater element is connected to an electric control switch.
It is designed to shorten choke duration at temperatures
above approximately 60 degrees. The switch serves sev-
eral purposes. Below 60 degrees it will provide the choke
heater with partial power or heat, allowing it to stay on
longer. Above 60 degrees it provides full heat to get the
choke off quicker. The switch temperature is controlled by
engine temperature and a small internal electrical heater.
To check the electrical heating element an ohmmeter is
used. Resistance of 4 to 12 ohms is normal; check specs
for particular application.
Some models use a 100 percent electric choke.
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TAMPER PROOF CHOKE
To meet federal regulations on tamperproofing, some
late models use rivets or breakaway screws to attach the
thermostatic choke coil and housing.
For a period of time, regulations required tam-
perproofing the choke pull-off linkage. On these units the
choke pull-off is spot welded to a housing which serves as
the mounting bracket and also a part of the tamperproof
enclosure.
The outside cover plate is riveted on to enclose the
choke pull-off link.
Fast Idle and Unloader
During the warm-up period it is necessary to provide a
tast idle speed to prevent engine stalling. This is accom-
plished by a fast idle cam connected to the choke shaft.
The choke trip lever contacts the fast idle cam. The fast
idle link attached to the throttle lever contacts the choke
trip lever and prevents the throttle valve from returning to a
normal warm engine idle position while the automatic
choke is in operation.
If during the starting period the engine becomes
flooded, the choke valve may be opened manually to
clean out any excessive fuel in the intake manifold. This
may be accomplished by depressing the accelerator
pedal to the floor mat and engaging the starter. The
unloader projection on the fast idle link will contact the
unloader lug on the choke trip lever and in turn partially
open the choke valve.
EGR Control
All vehicles since 1973 have used an EGR, or “Exhaust
a Recirculation,” system to lower emissions of nitrogen
oxides.
The EGR valve is controlled either by a port in the
throttle body above the closed throttle valve or by venturi
vacuum.
The ported EGR conrol takes advantage of throttle
valve position to open the EGR valve. At idle, the port is on
the atmospheric side of the throttle valve keeping the EGR
valve closed. As the throttle valves are opened, the port is
exposed to the manifold vacuum which opens the EGR
valve. |
The ported EGR uses a notched throttle valve to reduce
sensitivity for smoother EGR operation.
ЕСЯ
VENTURI
Some models use the venturi vacuum control system
whereby a vacuurn tap at the throat of the carburetor
venturi is used to provide a control signal.
OPERATING
LEVER
Wide Open Throttle Dump Valve
Some applications use a WOT, or “Wide Open
Throttle,” dump valve for the EGR system. The dump
valve will “kill” the venturi signal to atmosphere. The dump
valve is in series with the EGR venturi port in the car-
buretor and the amplifier. At wide open throttle, the arm on
the throttle shaft opens the dump valve cutting off the EGR
and giving full horsepower.
THROTTLE POSITIONER SOLENOID
On some applications the BBD carburetor uses a
throttle positioner solenoid as part of a catalyst
protection system. The system s function is to prevent
unburned hydrocarbons from entering the atmosphere
through the vehicle's exhaust system when the engine
is decelerated from a high RPM.
The solenoid works in conjunction with an electronic
speed switch and positions the throttle valves during rapid
deceleration to prevent over- rich mixtures contaminating
the catalytic converter.
The electronic speed switch senses the pulses from the
electronic ignition system. When the engine is operating
above approximately 2,000 R.P.M., the electronic speed
switch energizes the throttle positioner solenoid. On de-
celeration, the throttle positioner solenoid holds the
throttle valves to approximately 1,800 R.P.M. When en-
gine speed drops below 2,000 R.P.M., the throttle posi-
tioner solenoid is de-energized allowing the throttle valves
to close. Thus, the converter is protected from
overheating.
Some California units use a vacuum throttle positioner
which consist of an electronic speed switch, an electrically
controlled vacuum solenoid valve and a vacuum activated
positioner.
Its function and operation is the same as the solenoid-
type positioner above.
DASHPOT
Some applications use a slow closing throttle device
commonly called a dashpot. They are used to delay or
slow the throttle closing the last few degrees to prevent
engine stalling at the lower speeds and also to eliminate a
sudden peak of hydrocarbon emissions on sudden de-
celeration. At idle, the manifold pressure is very low and
results in good vaporization of the air-fuel mixture in the
intake manifold. When the throttle valve is opened, mani-
fold pressure increases. This increase in pressure in-
creases the boiling point of the liquid and prevents 100%
vaporization of the air-fuel mixture. During these periods
of high manifold pressures, there are some wet particles
of fuel clinging to the inside of the intake manifold which is
known as “wet manifold.” During sudden deceleration, the
manifold pressure goes back to a low pressure state, the
wet particles clinging to the inside of the intake manifold
go back to a vapor state and are taken into the engine as a
rich mixture. This is known as “manifold flash” and can
cause the engine to die out, especially at low speeds. The
dashpot slows the closing of the throttle the last few
degrees to give the engine time to clear itself of manifold
flash.
PURGE PORT
Starting in 1971 all outside carburetor vents had to be
routed to a cannister to prevent evaporative emissions to
atmosphere. A purge port has been added to the car-
buretor to purge the cannister of these fuel vapors. The
purge port is located above the throttle valve. As the
throttle valves open, the purge port is exposed to low
pressure which gives a predetermined air flow to scav-
ange these vapors from the cannister. |
Port relation is the position of the throttle valve relative
to the idle port. Anything that changes this relationship will
seriously affect idle, acceleration, EGR, spark and purge
timing. Proper idle adjustment for correct positioning of the
throttle valves is most important.
TRANSDUCER
TRANSDUCER AND GROUND SWITCH
Carburetors used on the lean bum engines use a
throttle position transducer and a ground switch. The
transducer is simply a device that changes mechanical
motion to an electrical signal. It consists of a coil enclosed
in plastic with a moveable iron core which is attached by
linkage to the throttle lever. Its movement and position is
always relative to throttle position and throttle movement.
The transducer signals the ESA, or “Electronic Spark
Advance,” computer the position and rate of change of the
throttle. The ESA then adjusts ignition timing to coincide
with throttle position and rate of opening.
The function of the ground switch is to signal the ESA
computer when the throttle valves are closed. The ESA
retards timing at closed throttle position.
Some models incorporate a grounding switch to contro!
the distributor solenoid. When the throttle valves are at
idle position, the grounding switch grounds the distributor
solenoid which retards ignition timing.
THE 0: FEEDBACK SYSTEM USING
VARIABLE AIR BLEEDS
Lomme
CLEAN |
AIR INLET |
HIGH
CIRCUIT PULSING
SOLENOID
|
LOW
SPEED
CIRCUIT
A
Carburetor Operation
The basic carburetor contains two fuel supply sub-
systems, the high speed system and the low-speed sys-
tem. The high-speed system meters fuel with a tapered
metering rod positioned in the jet by the throttle. Fuel is
metered into the main nozzle well where air from the
feedback controlled variable air bleed is introduced. Since
this air is delivered above the fuel level, it reduces the
vacuum signal on the fuel, consequently reducing the
amount of fuel delivered from the nozzle.
The idle system is needed at low air flows through the
venturi because there is insufficient vacuum at the nozzle
to draw fuel into the air stream. After leaving the main jet,
fuel is supplied to the idle system by the low-speed jet. It is
then mixed with air from the idle by-pass, then accelerated
through the economizer and mixed with additional air from
the idle bleed before being discharged from the idle ports
below the throttle. Air from the variable air bleed is intro-
duced between the economizer and idle bleed. This air
reduces the vacuum signal on the low-speed jet and
consequently the amount of fuel delivered to the idle
system.
The variable air bleeds change the pressure difference
which controls fuel flow thru the jets. |
Two types of air metering control are used on the BBD
0, feedback system. The stepper motor with air metering
pulse solenoid.
BBD WITH STEPPER MOTOR
The variable air bleeds consist of tapered metering pins
positioned in orifices by the stepper motor. This drive
mechanism moves the pins in defined steps in response
to signals from the oxygen sensor located in the exhaust
and processed by the electronic control unit. The stepper
motor moves the pins until the exhaust sensor indicates
- pins are used on some applications, while others use a
that the desired air-fuel ratio has been reached. Thus the
pin movement adjusts the air-fuel ratio to compensate for
changes detected in the exhaust gases.
AIR METERING
PIN-HIGH
SPEED CIRCUIT
Stepper Motor & Air Metering Pins
The digital linear actuator or stepper motor moves the
metering pins, 400 inch from full lean to full rich. Full
movement requires 100 steps at .004 inch per step.
Fast speed is 100 steps per second, slow speed is 12
steps per second.
During initial power-up of the stepper motor, the meter-
ing pins must be sent to an end stop to give the electronic
control unit a stable reference. The metering pins must
then be backed off to the desired position.
This initialization of the metering pins occurs on open
loop mode. When the ignition is turned on, and again
when the starter is engaged, the metering pins move
inward (rich position) 127 steps and outward (lean posi-
| tion) 35 steps. This locates the pins near the position to
give the average air-fuel ratio for complete fuel com-
bustion (stoichiometric ratio).
1 3 Pa RE INEA e TE CoD Eamets et vy a ON
Coil Windings
The stepper motor incorporates a unipolar winding
which has 2 coils wound on the same bobbin per stator
half for a total of 4 coils.
A threaded shaft provides linear movement which is
bidirectional. Movement and direction is controlled by
motor phasing sequence.
Checking Stepper Motor
To check stepper operation, remove the air cleaner
from the carburetor. The air metering pin for the high
speed circuit is visible looking into the air horn of the
carburetor.
Turning the ignition switch on should cause initialization
of the air metering pins. If no movement is observed,
check electrical connections and windings.
MOTOR PHASING SEQUENCE
~ EXTEND RETRACT
TERMINAL | TERMINAL
STP 213 [2 [1 |9 [4 |3 |2 |1
1 |6 |6 |- — | 1 |——|G |G
ЭГ Ге 129 |8/8/—1=
4 |6 | — | — | С 4 G —|I— G
_5 |6 [в [-|- |5 |-- Г Те Гб:
Checking Coils
Check each winding of the stepper motor by discon-
necting the wiring hamess from the stepper motor. With
an ohmmeter, check each coil by connecting to “C” ter-
minal and to ground terminal of each coil. There should be
78 ohms, plus or minus 25 ohms at room temperature.
Movement of the air metering pins can be accom-
plished by applying 12 volts to the “C” terminal and
grounding the coils as per the phasing sequence.
Air Management Switch
On some applications with Os feedback system, a
micro switch is incorporated which is operated by the
throttle shaft. It is part of the air management system and
makes contact at 25 degrees before wide open throttle.
When contact is made, it dumps the air pump air to
atmosphere.
BBD WITH PULSE SOLENOID
Some models use a pulse solenoid to control the vari-
able air bleeds. This eliminates the metering pins, as the
pulse cycle controls the air-fuel ratio.
The solenoid has only two positions of operation,
opened when energized to bleed air to both the high
speed and low speed circuits, or closed when de-
energized, cutting off the air bleeds.
CLOSED
T=100 MS FOR 10Hz OPERATION
Pulse Width Modulation
During normal operation, the solenoid goes thru one
open and one closed period in each cycle. The pulse
solenoid is a 10 Hz. frequency (10 cycles per second)
which adds up to 100 Mil/sec. per cycle.
Each cycle has a particular time period, “T”, from
beginning of one cycle to the next and is held constant
during operation (always 10 cycles per second).
During any one cycle, the solenoid is open for some
fractional period of time, “t". The duration of “t" can be
varied, thus varying the duty cycle and amount of air bleed
to the carburetor circuits.
100%: duty cycle means full air bleed for approximately
100 Mil/sec. per cycle. This duty cycle may be varied from
zero percent to one hundred percent.
Pulse width modulation of the air flow is controlled by
the solenoid duty cycle as signaled by the computer.
Specification:
Hesistance 22 + 1 ohms at room temperature
Checking Pulse Solenoid
Checking the pulse solenoid is very quick and easy.
With engine at operating temperature, merely place hand
on solenoid. If not pulsing, shut off engine and disconnect
pulse solenoid wires.
Check for open or shorted coil winding by using an
ohmmeter across the two blue wires. (The coil is not
grounded to the case). Should be 22 ohms resistance at
room temperature.
If winding checks good, momentarily flash 12 volts to
pulse solenoid to check armature movement.
A dwell meter can be used with the pulse solenoid to
give an overall indication of operation. The dwell reading
would be indicative of the ratio of “on” to “off” time which is
referred to as pulse width modulation. With engine
warmed up, place fast idle cam to obtain approximately
1200 R.PM. and check dwell reading. Closing the choke
valve slightly to richen air fuel mixture should give an
increase in dwell.
A command from the computer to “lean out” would give
a dwell reading between 30 to 60 degrees dwell, a rich
command would read between 0 to 30 degrees dwell. An
ideal reading would be between 28 to 32 degrees.
The dwell meter should always be set on the 6 cylinder
EXPLODED VIEW — AIR BLED DESIGN
PARTS LISTS
SWE OE =
FN AONSSSOEUSaRONS
FREFEIOTMMOoOOm >
>
Pin spring
Retainer
Spacer
Choke connector rod
Throtile connector rod
Pin spring (small)
Choke diaphragm
connector link
Hose
Choke shaft lever
Choke diaphragm screws
Choke diaphragm
assembly
Air horn screws (short)
Air horn screws (long)
Dash pot and bracket assy.
Air horn
Air horn gasket
Pump plunger assembly
Pump plunger washer
Pump plunger bushing
Pump plunger spring
Pin spring (plunger rod)
. Plunger shaft retainer
Pump arm screw
Pump arm
Venturi cluster screw
Venturi cover
Venturi cover gasket
. Venturi cluster assembly
Veniuri cluster gasket
Pump intake check ball
(large)
Pump discharge check
ball (small)
Slep-up piston plate
SCrEw
Step-up piston plate
step-up piston rod (2)
Step-up piston spring (2)
Step-up piston gasket
Needle & seat assembly
Float lever pin retainer
Float & lever assembly
Float lever pin
Main jets
Compensator valve screw
Compensator valve cover
Compensator valve
Compensator gasket
Main body casting
Body flange screw
Body flange gasket
Idle limiter cap
Idle mixture screw
Throttle speed screw
Flange assembly
Retainer (2)
Washer
Pump arm screw
Pump arm
Cover plate screw
Cover plate
Vent valve spring
Vent valve
Cover plate gasket
Pump plunger
Pump plunger spring
EXPLODED VIEW — SOLID FUEL DESIGN
PE
on Jas Ca) I =k
ЗОВ.
37.
ЗВА.
39,
41.
42.
43.
45.
46.
47.
47A.
48.
PARTS LISTS
Choke shaft lever screw
Choke shaft lever
Choke pull-off rod
Choke pull-off bracket
screw
Choke pull-off and
bracket
. Choke pull-off
housing—if equipped
. Choke pull-off housing
rivels—Iil equipped
“E” retainer
Throttle connector rod
Fast idle cam screw
Fast idle cam
. Fast idle rod
. Dust cover screw
Dust cover
. Dust cover gaskel
Pump and metering
rod arm screw (2)
Pump and metering
rod arm washer (2)
Pump counter shaft
Metering rod arm
Pump arm
Pump Ҥ" link
. Vacuum piston assembly
. Metering rod (2)
. Vent valve grommet
seal—if equipped
Choke pull-off hose
E.G.R. Dump valve hose
. Bowl cover screw
. Carburetor identifica-
tion tag
. Bowl cover and bracket
Screw (2)
. Bowl cover (2)
Bowl cover gasket
Solenoid & bracket
. Vacuum modulator
Transducer, bracket &
idle ground post—if
equipped
. Idle enrichment cover
screw (3)
. Idle enrichment cover
idle enrichment cover
spring
Idle enrichment cover
gasket
Idle enrichment
diaphragm
Plunger spring
Plunger assembly
. Intake check ball (large)
(See note 5, Ро. 4)
Vacuum piston spring
. Needle, seat, and gasket
Bafile
Float pin retainer
Float
. Float pin
Main metering jets (2)
Venturi Cluster
screw (2)
Venturi cover assembly
Venturi cover assembly
(AIt.)
Venturi cover gaskel
285
SsRSN
GT
=]
se
=
=
Venturi cluster assembly
Venturi gasket
Discharge check ball
(small)
Body flange screw
Body flange
Body flange gasket
Main body casting
E.G.R. Dump valve
bracket screw
. E.G.A. Dump valve
. Limiter cap (2)
idle mixture screw (2)
Idle mixture screw
spring (2)
. Idle Mixture screw
plugs (2)
—
ADJUSTMENTS — AIR BLED DESIGN
FLOAT SETTING
Invert casting and hold finger against float fulcrum pin
retainer to assure fulcrum pin is bottomed in its guide
slots. Measure the dimension as shown in specifica-
tions from surface of fuel bowl to the top of crown at cen-
ter of each float (1955-56 at outer ends of float). To ad-
just bend lip of float.
NOTE: Never allow the needle to be pressed into seat
~ when adjusting.
|
Г
|
|
FAST IDLE TYPE | — OFF ENGINE
Open throttle valve slightly and hold choke valve fully
closed to allow fast idle cam (in piston housing) to rotate to
fast idle position. The dimension between lower edge of
throttle valve and bore of casting should be as specified.
To adjust, bend connector rod (C).
TYPE ll — OFF ENGINE
Place fast idle screw (A) on the index mark (or highest
step) of fast idle cam and adjust the screw to the dimen-
sion as specified, between lower edge of throttle valve and
edge of casting.
TYPE Ill — ON ENGINE
With engine running at operating temperature, place
fast idle screw on step of cam as shown in specifications,
then adjust fast idle screw to RPM specifications.
UNLOADER
Hold throttle valves wide open and close choke valve
as far as possible without forcing. The dimension be-
tween top edge of choke valve and inner wall of air horn,
should be as specified. To adjust (1955 and early 1956
carburetors) bend trip lever arm in housing; (late 1956
and later -see insert) bend unloader arm (B) on throttle
lever.
CLOSING PRESSURE ON
CHOKE SHAFT LEVER =
CHOKE er
OPERATING
CAUGE OR DRILL
CHOKE VACUUM KICK — IF EQUIPPED
Press Diaphragm stem inward until diaphragm is bot-
tomed on 1964 carburetors; 1965 and later, press dia-
phragm plunger (not stem) to bottom diaphragm to allow
diaphragm stem internal spring to be compressed by
extending the stem as choke valve is moved toward the
closed position to obtain the proper dimension between
top edge of choke valve and wall of air horn. To adjust to
specifications, open or close the “U" bend of choke oper-
ating link.
NOTE: Optional method of bottoming diaphragm is to
apply at least 10” of vacuum from an outside source to
diaphragm assembly.
CHOKE HOUSING
COVER AND
THERMOSTAT
COIL INDEX
AUTOMATIC CHOKE
Carburetors equipped with integral choke. Rotate cover
against spring tension until specified mark on thermostatic
coil housing is aligned with mark on choke piston housing.
TYPE1& 2 |
PUMP
With throttle valves at curb idle and throttle connector
rod (A) in center hole of throttle lever and inner hole of
pump arm (unless otherwise noted in specifications).
TYPE |
The dimension (B) from surface of casting to top of
plunger shaft should be as listed in specifications. To
adjust, bend connector rod (A).
TYPE II
The pin spring should be in center groove of plunger
shaft to support vent valve for standard setting, unless
otherwise noted in specifications.
NOTE: Change pin in accord with pump stroke.
To adjust, bend connector rod (A).
TYPE III
The retainer should be in center groove of plunger
shaft. The dimension from the air cleaner gasket sur-
face of air horn to top of plunger rod, should be as
specified. To adjust, bend connector rod (A).
DASHPOT — IF EQUIPPED
With throttle valves at curb idle, hold dash pot stem
fully depressed. Loosen lock nut and adjust dashpot in or
out of bracket to obtain 1/16" between diaphragm stem
and throttle lever tang.
Tighten locknut.
IDLE SPEED AND MIXTURE
Non-Emission Carburetors
Turn throttle speed screw in until throttle valves are |
opened slightly. Start engine and allow to warm up |
thoroughly. Turn mixture screws either way until the best |
idle is obtained. Readjust throttle speed screw to 450-500 |
RPM and again check mixture screws. 1968 and later |
carburetors see tune up decal in engine compartment for
the proper RPM.
Emission Carburetors
Follow idle mixture adjusting procedure as outlined in
car manufacturer's service manual. If not available, make
temporary adjustment as follows:
1. Check ignition timing.
2. With engine at normal operating temperature, air
cleaner installed where possible, and all transmissions
in neutral.
3. Tum throttle speed screw for speed of 500-550 RPM.
For (C.A.P) carburetors tum throttle speed screw to
700 RPM for Manual Transmissions, and 650 RPM for
Auto Transmissions. For 1968 and later carburetors
see tune up decal in engine compartment for specified
RPM.
4. Tum idle mixture screws for the highest RPM using a
tachometer.
5. Readjust throttle speed screw if necessary.
6. Tum each mixture screw clockwise (leaner) slowly, to
obtain 10 to 20 RPM drop with each screw. Then turn
each screw 1/4 turn counterclockwise (richer) for final
adjustment.
LIMITER a”
LIMITER CAP INSTALLATION — IF
EQUIPPED
If the original limiter caps have been removed from the
carburetor, the new service idle limiter caps must be
installed after properly adjusting the idle speed and mix-
ture screw to comply with existing State and Federal
regulations regarding Exhaust Emissions.
Soak caps in hot water for a few minutes to aid in
installation. Place caps on mixture screw heads and press
firmly using care not to tum mixture screws when forcing
caps in place, with the tab in the maximum counter-
clockwise position against the limiter stops.
ADJUSTMENTS — SOLID FUEL DESIGN
FLOAT SETTING
Hold float lip (A) against seated needle lightly while
holding retainer (B) in bottom of guide slot. The dimen-
sions between top of float (at center) and top of bowl
should be as listed in specifications. To adjust remove
float and bend lip (A).
NOTE: Never allow the needle to be pressed into seat
when adjusting.
METERING ROD
Back out the throttle speed screw to allow the throttle
valves to close completely. Loosen the rod lifter lock screw
(B). Fully depress the step-up piston (C) to bottom the
metering rods. Apply light pressure on rod lifter tab (D)
until the lip of tab contacts piston plate. Tighten screw (B).
B
SCALE 1:
PUMP
Turn curb idle screw two full turns clockwise after it
just contacts stop, then hold throttle closed. Using a TT”
scale, measure the dimension from the top of ac-
celerator pump shaft to the top of bowl cover. It should
be as shown in specifications. To adjust, loosen pump
arm lock screw (B) and revolve pump arm (C). Tighten
screw (B).
BOWL VENT — IF EQUIPPED
Turn curb idle screw two turns clockwise after it just
contacts stop. With throttle held closed, a 3/32 drill should
fit between the grommet seal (A) and its seat, with only a
slight drag on the drill. Drill gauge must be positioned to
touch the roll valve pin (B) while gauging the valve. To
adjust, bend tang (C).
FAST IDLE CAM
Place fast idle speed adjusting screw (A) on the second
highest step of cam. Apply a light closing pressure on
choke lever (B) to move the choke valve toward the closed
position. The dimension (C) between the upper edge of
choke valve and air horn wall should be as listed in
specifications. To adjust, bend connector rod (D).
E.G.R. DUMP VALVE - IF EQUIPPED
With throttle valves held wide open and plunger stem
fully depressed, the dimension (A) between operating
lever and valve body should be 1/32". To adjust, loosen
locknut (B) on body and tum valve (C) in or out to proper
dimension. Tighten locknut.
TO VACUUM
SOURCE
CHOKE PULL-OFF
Use an outside vacuum source to retract diaphragm |
stem fully. Apply a light closing pressure to choke lever |
(A), to move the choke valve toward the closed position |
as far as possible without forcing. The dimension (B) |
between the upper edge of choke valve and wall of air |
horn should be as listed in specifications. To adjust, open
or close the “U” bend of connector rod at (C).
COVER AND THERMOSTAT | В
COIL INDEX MARK
(Pas UNLOADER
#8 117 With throttle in wide open position, apply light closing
MT | == / E pressure on choke lever (A) to move choke valve toward
> E the closed position. The dimension (C) between the upper
Z =P -e edge of choke valve and wall of air horn should be as listed
in specifications. To adjust, bend tang (8) on throttle lever. |
NU
MN INDEX MARK
A
ATT m
AT
AUTOMATIC CHOKE
Rotate cover against spring tension until specified mark
on thermostatic coil housing is aligned with mark on choke
piston housing.
C
mo HIGH ALTITUDE ADJUSTMENT - IF
т) EQUIPPED
: Turn screw (A) counterclockwise from seated position
for high altitude operation. For sea level operation tum |
eS. screw (A) clockwise to seal venturi cluster bleed cap (В). |
Refer to decal in engine compartment for proper
specifications.
IDLE STOP
SOLENOID MM
2 с”
SOLENOID “>
WIRE
IDLE SPEED AND MIXTURE
Use exhaust analyzer if available. If not available make
temporary adjustment as follows:
1. Refer to the "Emission Control Decal” in engine com-
partment for the proper engine RPM.
2. With engine at normal operaing temperature, choke
fully open, air cleaner installed, automatic transmission
in neutral, and air conditioner turned off.
3. Connect a tachometer and turn idle speed screw (A) or
if equipped with the idle stop solenoid, turn solenoid
speed screw (B) to the specified engine RPM, with the
solenoid wire connected to energize the solenoid.
NOTE: The 1975 models equipped with the Catalyst
Protection System will include a throttle solenoid posi-
tioner, and can be identified by a printed decal on the
solenoid which states DO NOT USE solenoid or screw to
set idle speed.
4, Turn the mixture screws (C) counterclockwise (richer)
until a loss if engine RPM is indicated on tachometer.
Turn the mixture screws (C) clockwise (leaner) until the
highest RPM is obtained, then continue turning clock-
wise until engine RPM starts to decrease. Turn the
mixture screws counterclockwise (richer) until the lean
best idle setting is obtained. Readjust speed screw if
needed. If equipped with the idle stop solenoid, and
with engine running, turn speed screw (A) inward until
end of screw just touches stop, now back off one full
tum to obtain low speed setting.
1977 AND LATER IDLE MIXTURE
AND SPEED ADJUSTMENT
Refer to decal in engine compartment for proper pro-
cedures and specifications. On models equipped with idle
mixture screw plugs install replacement plugs.
LIMITER STOP(S)
“LIMITER CAP(S)
LIMITER CAP INSTALLATION — IF
EQUIPPED
The new idle limiter caps must be installed, after prop-
erly adjusting the idle speed and mixture to comply with
existing State and Federal regulations regarding Exhaust
Emission.
Soak caps in hot water for a few minutes to aid in |
installation. Place caps on mixture screw heads and press
firmly to seat, with the tab in the maximum counter-
clockwise position against the limiter stops.
FAST IDLE — ON CAR
“With the fast idle speed screw (A) placed on the second
highest step of fast idle cam, turn the screw to obtain the
RPM as listed in specifications.
THROTTLE DO NOT USE
POSITIONER SOLENOID OR SCREW
SOLENOID
= TO SET IDLE SPEED
5
| APPLY BATTERY f(D
| 3 = \
VOLTAGE — ro
LEVER
THROTTLE POSITIONER SOLENOID —
IF EQUIPPED
(Catalyst Protection System)
1. Engine off, disconnect the solenoid wire and hold
throttle wide open. Apply battery voltage with a jumper
lead to solenoid wire. The solenoid stem should extend
its full length and maintain its extended position. If it
does not, replace unit. Remove the jumper lead from
solenoid wire and release throttle.
2. Connect a tachometer, start engine, again apply bat-
tery voltage, with jumper lead to solenoid wire. Adjust
engine speed screw (D), if needed, to approximately
1500 RPM, allow time for O.S.A.C. valve to provide
vacuum spark advance and engine speed to stabilize.
Disconnect the jumper lead and reconnect the sole-
noid wire.
3. Accelerate engine manually to approximately 2500
RPM and release throttle. Engine should retum to
normal idle.
VACUUM
a THROTTLE
| POS ITIONER
THROTTLE
LEVER
VACUUM THROTTLE POSITIONER —
IF EQUIPPED
(Catalyst Protection System)
1. Accelerate engine manually to speed of approximately
2500 RPM.
2. Loosen nut (A) and rotate vacuum throttle positioner
until positioner stem just contacts at tang (C) on throttle
lever. Release throttle, then slowly rotate the solenoid
throttle positioner to decrease engine speed until a
sudden drop in speed occurs (above 1000 RPM). At
this point continue adjusting the vacuum positioner in
the decreasing direction 1/4 additional tum and tiahten
nut (A).
| RPM ADJUSTMENT
| VACUUM UNIT IDLE RPM |
| ENERGIZED ADJUSTMENT |
\ e \ L |
X A
- J
.
O
o O
APM ADJUSTMENT
SOLENOID ENERGIZED !
VACUUM
HOSE
SOL-VAC
Three adjustments are required and must be made in
the proper sequence.
1. Disconnect vacuum hose from solenoid vacuum unit
and plug hose. Also disconnect the electric wire to the
solenoid. Adjust normal curb idle with R.P.M. screw.
2. Using a hand vacuum pump, apply vacuum to the
solenoid vacuum unit and adjust to the proper R.P.M.
with the screw located on the throttle lever. Remove
pump.
3. Energize solenoid and adjust R.P.M. to specifications
using the adjusting screw on rear of solenoid.
DASH POT
DASHPOT — IF EQUIPPED
Loosen lock nut (A). Start engine and connect a tac-
hometer. Position throttle lever to 2500 RPM. Adjust
dashpot until the stem just contacts tang at (D) on throttle
lever. Tighten nut (A). Check to make sure engine retums |
to idle after making this adjustment.
TRANSDUCER — IF EQUIPPED
To adjust the transducer, measure distance between
outer portion of transducer and transducer mounting
bracket. Tum transducer clockwise or counterclockwise to
obtain distance as specified.
CORPORATION
~ PLYMOUTH « DODGE
CHRYSLER « DODGE TRUCKS
76 November
Reference Book
TWO BARRELS
FOR
SIX CYLINDERS
MASTER
WHITH
SERVICE CONFERENCE
ONE
GOOD CARBURETOR
DESERVES ANOTHER
For the past several years we've had the famil-
iar and reliable Carter two-barrel carburetor
for our 318 engines. Now, for 1977, this same
basic carburetor (with modifications) is also
installed on the 225 Super Six powerplants.
Although both Carters look alike, there are im-
portant differences between them that you
should know about since they affect service.
Some are major, others of a minor nature. For
one thing, the Carter for the Super Six and the
Fig. 1 — The new Super Six carburetor far 1977
ba =F
Carter for the 318 engine are not interchange-
able. Poor driveability, poor fuel economy, and
reduced performance will result if this is at-
tempted.
There are a number of other features regard-
ing the new Super Six carburetor that are also
discussed in this Reference Book. And, in the
following pages, we've also covered the
reasons for these changes and step-by-step
procedures for making all external checks and
adjustments. As you may well know, many of
the engineering modifications you'll find in
this Super Six carburetor have been incorpo-
rated simply because control of exhaust emis-
sions is so critical in our drive for clean air.
CONTENTS
NEW FEATURES :== co. .e=vzm=..> >... 1
EXTERNAL ADJUSTMENTS ........ee.... 4
WET FLOAT LEVEL SETTING ............ . 9
ADDITIONAL CARBURETOR
INFORMATION :.e..e.eecesacmnvorrenaao 11
| | NEW
_| FEATURES
THE THROTTLE BODY RESTRICTORS
One of the more important changes made to
the throttle body on the Super Six carburetors
Is the addition of idle mixture restrictors.
These restrictors are simply small brass plugs
with a calibrated drilled hole. With the throttle
body separated from the main body, you'll find
them pressed into position just above the idle
discharge ports and in line with the idle mix-
ture screws.
Fig. 2 — Idle mixture restrictors are calibrated
These restrictors act much like main metering
jets. That is, only so much fuel mixture can
flow through the drilled passage, regardless of
how far the idle mixture screws are turned
outward. On the other hand, leaner idle speed
mixtures can be easily achieved by turning the
mixture screws inward until you get the proper
HC and CO emission readings called for on the
underhood Emission Label.
CAUTION: Under no circumstances should
these restrictors be removed or the:calibrated
drilled passages enlarged.
THROTTLE BORE AND
VENTURI DIMENSIONS
Although the throttle body bore for the 1977
Carter Super Six and 318 engines are identical
(1-7/16"), thère is a difference in the main body
venturi size. Since the Super Six has less cubic
inch displacement than the 318, the main ven-
turi needed for the Super Six is smaller (1-1/16")
than the venturi for the 318 (1-3/16"). This is one
of the primary reasons for non-interchange-
ability of the two-barrel Carter for the Super Six
and the 318 engines.
ЗУРЕК STE
(ITR ИВАН
Fig. 3— Throttle bore size is identical
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318 TWO-BARREL
Na АЛ.
['W(-BARREI
Fig. 4 — Notches provide source for vacuum signals
(1)
THROTTLE BODY NOTCHES
Both the throttle body for the Super Six Carter
carburetor and the Carter two-barrel carburetor
for the 318 for 1977 have notches cast into the
underside section. Prior models of the Carter
two-barrel do not have notches. Through these
notches, intake manifold vacuum is applied to
the vacuum kick diaphragm and to the inlet air
door diaphragm which is part of the heated air
system.
FLANGE GASKET
This design change in the throttle body dictated
the need for a new carburetor flange to intake
manifold gasket. If the 1977 gasket is used on a
1976 or prior model two-barrel Carter car-
buretor, the vacuum source for vacuum kick
and the heated air system will be blocked off.
NN 318
TWO-BARREL NO
Fig. 5 — The Super Six “bathtub'' design base gasket
In other words, remember to use the solid gas-
ket (without notches) with the carburetor that
does have notches in the throttle body. And,
use the gasket (with notches) on Carter two-
barrel carburetors that do not have notches in
the throttle body.
IDLE MIXTURE SCREWS
For many years idle mixture screws have had
very little change. But now for 1977 there's a
difference.
Although the screw threads remain identical in
size and pitch, the tip of the new mixture screw
has a slimmer, more gradual taper and the end
of the tip is not pointed, These new mixture
screws are not interchangeable with those
(2)
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1977
Fig. 6 — 1977 mixture screws have a slimmer needle tip
used in prior model Carter two-barrel car-
buretors.
If a prior model mixture screw is installed in a
1977 Carter two-barrel carburetor, damage to
the idle ports can result because internal
machining of the throttle body is also different
to accommodate the new taper of the mixture
SCrews.
AIR HORN
There are two differences in the air horn used
on Carter Super Six carburetors as compared
to the one used on the 318 engine. First, the
choke valve for the Super Six has more offset
than the one used with the 318 engine. Because
of this, engineers increased the thickness of the
air horn inner wall. By increasing the amount of
choke offset, air velocity rushing past the par-
SUPER SIX
TWO-BARREL
[IRL ON
Fig. 7 — The Super Six air horn wall is thicker
SUPER SIX
TWO-BARREL
Fig. 9 — Air velocity opens choke valve wider
tially closed choke valve when the throttles are
opened during cold engine operation, causes
the choke to open slightly more than it would if
there was less choke offset, As a result, the en-
gine receives a leaner warm-up mixture under
the engine operating condition described in
order to help keep emissions down.
VACUUM STEP-UP PISTON
While we're on the subject of emission control,
engineers have made some minor but signifi-
cant modifications in the vacuum step-up pis-
ton assembly for the Super Six carburetor.
Two things have been changed: the plastic
lifter tabs are somewhat larger and the dimen-
sion between the upper and lower stops of the
step-up piston is reduced. As a result, the vac-
318 SUPER SIX
TWO-BARREL [IEE
Fig. 10 — Note difference in step-up piston lifter tabs
uum step-up piston now has a restricted
amount of free travel as compared to the 318
version.
LIMITED STEP-UP PISTON TRAVEL
Now, when manifold vacuum drops during part
throttle, heavy-load operation, the weak vac-
uum signal applied to the piston permits
the piston spring to push the piston and meter-
ing rods upward only a slight amount. This
limited piston travel helps keep mixture levels
from becoming excessively rich during the en-
gine operating condition described. Again,
there is no interchangeability between the vac-
uum step-up piston assembly used on the
Super Six Carter carburetor and the one for the
318. Otherwise, flat spots, hesitation, and pos-
sibly stallout will result.
==
NE
TWO-BARREL TWO-BARREL
Fig. 11 — Vacuum step-up piston stops are different
O
2 | EXTERNAL
A WORD ABOUT LINKAGE |
The external operating linkage connecting the
various cams and levers that are part of the
Carter two-barrel carburetor are purposely de-
signed to operate in somewhat “loose” man-
ner. The basic reason is to ensure unrestricted
“smooth” operation over many thousands of
miles even when dirt or gum deposits accumu-
late on the linkage. Because this "looseness’
Is ‘built in” to the carburetor design, some
Service Technicians feel that if the setting is
‘close-enough’ it's okay to leave it alone. But
remember, under all engine operating condi-
tions, this “slack” is taken up so that the set-
tings are correct. As a result, you can easily
see the need for PRECISE and ACCURATE set-
tings. And, of course, misadjustments of the °
carburetor linkage can upset emission levels
almost faster than any other component on the
engine. Therefore, trying to invent your own
specifications or making the adjustments
without the proper special gauges can only
lead to driveability problems.
Fig. 12 — Special gauges eliminate "guesswork"
®
ADJUSTMENTS
If you use “guesswork' you might just as well
call the whole thing off.
Lets agree on one thing .. . carburetors need
PRECISE adjustments in order for the engine to
deliver GOOD DRIVEABILITY and of course
provide good customer satisfaction.
By the way, all of the following external ad-
justments can be made when the carburetor is
on the bench or on the car, including the
“wet check of float level. In either case, it is
suggested that you perform these adjustments
in the order in which they are presented in this
Reference Book.
CHECKING FAST IDLE CAM POSITION
Before making any checks or adjustments to
the carburetor, first get the carburetor model
number stamped on the small tag attached to
the air horn. Then refer to the Service Manual
in order to select the right gauges you'll need.
The first check should be made to make sure
the fast idle cam position is correct. Here's why:
Since the choke valve and the fast idle cam are
linked together, we must be certain that at every
stage of choke valve opening the cam is re-
positioned properly in order to insure correct
engine RPM all during engine warm-up.
THE SECOND HIGHEST STEP OF THE CAM
To make this check, first place the fast idle
speed screw on the high step of the cam. Then,
carefully rotate the fast idle cam by hand so that
the fast idle speed screw slides and drops down
to the SECOND HIGHEST STEP.
With the choke coil rod disconnected, push the
choke valve towards the closed position. While
Fig. 13 — Fast idle screw rests on second step of cam
maintaining closing pressure on the choke
lever, insert the correct gauge between the air
horn wall and upper edge of the choke valve.
You should feel a slight drag on the gauge as
you move it up and down.
Fig. 14 — Checking fast idle cam position with gauge
CHOKE VALVE CLEARANCE
If there is too much clearance (no drag on the
gauge) then the connector rod length will have
to be INCREASED.
Increasing the rod length DECREASES the
choke valve opening. Now, if on the other hand,
the choke valve has too little clearance (gauge
cannot be inserted between the choke valve
and the air horn wall), then the connector rod
length will have to be DECREASED. Bend the
rod as necessary until there's a slight drag on
CLEARANCE
CHANGES ERE
M с
e по”
Fig. 15— Bend carefully to change choke valve clearance
the gauge, as it is moved up or down. Recheck
your adjustment so that it is set right to specifi-
cations.
Too little or too much choke valve clearance
caused by the connector rod being too long or
too short has a direct effect on driveability dur-
ing engine warm-up. With the choke valve
opening not synchronized with throttle valve
opening, stumbling, stalling and engine misfir-
ing generally results.
CHECKING VACUUM KICK DIAPHRAGM
When you are certain that the fast ¡dle cam posi-
tion is set accurately, proceed to the check of
vacuum kick. Of course, make sure the vacuum
kick diaphragm is not punctured, ruptured or
torn. You can do this by disconnecting the vac-
uum hose at the carburetor. Then, push the
stem of the diaphragm into the metal case so
that it bottoms. While holding the stem bot-
tomed, place your finger tight against the open
end of the vacuum hose and release finger
pressure on the vacuum kick stem.
If the diaphragm is not damaged or ruptured,
the stem will remain bottomed in the metal
case. Any small leakage in the diaphragm will
cause the stem to release and move outward.
You can also check the vacuum diaphragm
condition by applying vacuum through a vac-
uum pump to the diaphragm. At about fifteen
inches of vacuum the stem should hold its
“pullin” position for at least 30 seconds or
more. If not, then install a new vacuum kick
(5)
Fig. 16 — Checking vacuum kick diaphragm condition
diaphragm assembly before you begin the vac-
uum kick check.
GAUGE CHECK VACUUM KICK
Now, select the vacuum kick gauge called for in
the Service Manual. Then with a vacuum hand
pump connected into the vacuum diaphragm
hose, pump up about fifteen inches of vacuum.
Open the throttle to clear the fast idle cam and
release the throttle. Close the choke by hand by
applying pressure to the choke lever.
БАШСЕ
Fig. 17 — Checking vacuum kick clearance with gauge
As you're doing this, the center of the vacuum
kick stem will pull out of the diaphragm with
little resistance. When the small internal spring
becomes fully compressed and you feel resis-
tance to further closing of the choke, STOP
RIGHT THERE.
(5)
If too much pressure is applied to the choke
lever when closing the choke valve, the dia-
phragm will become overextended causing the
choke valve to close more than it should.
As a rule of thumb . . . take it easy on choke
valve closing. Never use excessive force. Once
the diaphragm stem extends and you feel a re-
sistance to further movement of the stem, hold
that position and make the gauge check.
SIEM
AA
Fig. 18 — Vacuum kick diaphragm extended without farce
Using the correct gauge, insert it between the
choke valve and the inner wall of the air horn.
Excessive clearance causes too lean a fuel/air
mixture and results in engine stalling. On the—
other hand, too little clearance between the
gauge and the air horn wall will cause exces-
VALVE
Fig. 19 — Vacuum kick clearance correctly set
sively rich mixtures which results in high
exhaust emissions during engine warm-up.
CHANGING VACUUM KICK CHOKE
VALVE CLEARANCE
To increase the amount of choke valve clear-
ance for vacuum kick, CLOSE the "'U'"-shaped
link. To decrease the clearance, OPEN the link
using a wide-bladed screwdriver. Once the
vacuum kick adjustment is correct, reconnect
the choke rod. Now for the Choke Unloader.
CHECKING THE CHOKE UNLOADER —
WIDE OPEN KICK
This is often called the "Wide Open Kick"
check. When properly set and throttle valves
are wide open, the right amount of extra intake
air enters the cylinders during engine cranking
to help clear a flooding condition that is usually
caused by improper starting procedures during
cold engine start-up.
With the throttle valves held firmly in the wide-
open position (and finger pressure holding the
choke valve towards the closed position), insert
the proper gauge between the choke valve and
the inner wall of the air horn.
The gauge should have a slight drag as it is
moved up and down. If the gauge falls through
the gap without any drag or if it cannot be in-
serted without forcing the choke valve to open
wider, then the tang on the throttle shaft will
have to be bent up or down a slight amount.
You may have to do this several times in order
to get the setting correct,
11]: N
Fig. 20 — Checking choke unloader setting with gauge
CHECKING VACUUM STEP-UP PISTON
This check must be made with the throttle
valves completely closed. To do this, back off
the idle speed screw until the tip of the screw
clears the idle speed stop. Press downward on
the step-up piston assembly and loosen the
plastic lifter lock screw about one turn. While
maintaining downward pressure on the step-up
piston, also push the outer tab of the plastic
sleeve downward as far as it will travel, Tighten
the lock screw.
| ||
ME:
M N С
> _ SCREW
*
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Fig. 22 — Setting the vacuum step-up piston
That's all there is to setting the vacuum step-up
piston.
NEVER CHANGE HEX HEAD
SCREW ADJUSTMENT
As a word of caution, never attempt to change
the factory setting of the vacuum step-up piston
@
hex-head screw because performance can be
seriously affected. Calibration of this assembly
is accomplished by using special equipment
during production and cannot be duplicated in
the field.
RT
CHANGE
FACTORY EN
SETTING 7 N
SE
Fig. 23— Never change vacuum milan, up piston screw
CHECKING ACCELERATOR PUMP STROKE
This check must be made with the throttle
valves set in the curb idle position. To do this,
back-off the idle speed screw until it clears the
idle speed stop, then turn the curb idle speed
screw inward until it just touches the idle stop.
Give it TWO additional turns inward. This will
place the throttle valves in an approximate curb
idle position. Now, measure the distance be-
tween the top of the pump shaft and the top
surface of the air horn casting, without the
gasket.
TOP OF
IETF
O
Fig, 24 — Measuring accelerator pump stroke
The vertical section of the | scale should
touch the top of the pump shaft while the hori-
zontal crosspiece should be resting on the cast-
ing. A pump shaft set down too far reduces
pumpstroke travel.
Fig. 25 — Accelerator pump shaft set down too tar
As a result, too little fuel is discharged on ac-
celeration causing flat spots and hesitation. On
the other hand, a pump shaft set too high will
cause too much fuel to be discharged on ac-
celeration. Fuel economy suffers. In either
case, loosen the small lock screw on the plastic
sleeve and rotate the sleeve as necessary until
the pump shaft height is set to the specifica-
tions called for in the Service Manual. Once the
setting is correct, hold the position and snug
the screw down tightly. Recheck measurement.
NICT
JUD |
alfil la
ТО IGHIN +
Fig. 26 — Accelerator pump shaft set correctly
WET
(3 [FLOAT LEVEL
SETTING.
Checking the float level is not generally done
when performing the external checks of link-
age adjustments. However, depending upon
the driveability problem (or customer com-
plaint), you can make this check either by the
dry method or the "wet" method. The “wet”
method can be performed on-the-car or on-
the-bench. After removing the vacuum step-up
piston assembly, disconnect the accelerator
pump rod, the fast idle cam connector rod and
the thermostatic coil connecting rod. Separate
the vacuum kick hose from its connection at
the throttle body. Remove the air horn by lifting
straight upwards. With your fingers, push the
floats down to the bottom of the float bowl.
Fig. 27 — Allowing extra fuel to enter the float bowl
Extra fuel will enter the float bowl because of
pump pressure maintained in the line. If this
“wet method is performed on-the-bench, pour
enough gasoline into the float bowl! to create an
unusually high fuel level. By doing this, you can
make sure the fuel level is extra high so that the
floats press the needle snug against its seat.
With a higher than normal fuel level you'll get a
more accurate check of true float height.
CHECK FUEL INLET FITTING TIGHTNESS
Before checking float height, make sure the
Fig. 28 — Tightening inlet fitting changes float height
fuel inlet fitting is tight. Specifications call for
200 inch-pounds. This may seem too simple to
even mention but when you realize there's
about a ten-to-one ratio of movement between
the float needle and the floats, you can see why
this check is important. As an example: if there
Is only FIFTEEN THOUSANDTHS (.015) of an
inch of inward movement of the float needle
when the inlet fitting is tightened, the float
height will change almost ONE-HUNDRED AND
FIFTY THOUSANDTHS (.150").
Incidentally, for maximum protection of the
brass inlet fitting (the line fitting as well), use
two flare-nut-type wrenches to avoid crushing
or distorting either one.
MEASURING FLOAT HEIGHT
Using a “T" type of metal scale, measure the
distance between the metal surface of the
main body casting and the crown section of
each float . . . at their centers. As you are doing
this, press down on the float pin retainer so
that you get an accurate check. If the floats are
sel too high or too low, remove the float baffle,
then push the floats down to the bottom of the
float bowl and carefully bend the metal float
tab one way or the other as needed.
®
Fig. 28 — Be sure to press down on float pin retainer
Recheck the setting several times to be sure
the floats are set according to Service Manual
specifications.
EFFECTS OF HIGH/LOW FUEL LEVEL
If the floats are set too high, the level of fuel in
the main discharge nozzles will also be too
high. As a result, an excessive amount of fuel
will be discharged during normal driving con-
ditions, and fuel economy will suffer.
MAIN DISCHARGE
NOZZLE
On the other hand, if the float level is set too
low, the level of fuel in the main discharge
nozzles will also be too low. If this is the case,
the fuel must lift and travel a greater distance
before discharging into the intake air stream.
The result will be excessively lean mixtures
and driveability problems.
CHECKING THE FLOAT NEEDLE
As you know, the float needle tip Is a synthetic
rubber material to help ensure a tight seal
©
MAIN DISCHARGE
NOZZLE
y
Fig. 31 — Low float level causes excessively lean mixtures
against fuel pump pressure when the fuel level
reaches the correct height in the float bowl.
Some swelling of the synthetic rubber may
occur because of the chemical composition of
some fuels on the market. When swelling oc-
curs the fuel level lowers because the floats
are lowered. But, if there is no damage to the
float needle tip (no cuts or scoring), it is not
necessary to install a new needle and seat as-
sembly. Simply reinstall the original needle
and seat and reset the float level as called for
in the Service Manual specifications. Once the
float level is set to the correct height, reinstall
the float baffle and make sure the floats move
freely.
NORMAL APPEARANCE
a it
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F = J
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EXAGGERATED APPEARANCE
Fig. d2 — Some swelling of needle tip may occur
Reinstall the air horn with a new gasket. Re-
connect all the linkage rods, the vacuum Kick
hose, and tighten the cover screws. Reinstall
the vacuum step-up piston and metering rods,
and reset as described earlier.
4 | ADDITIONAL
4 | CARBURETOR
— | INFORMATION
Although this Reference Book is primarily con-
cerned with the Carter two-barrel carburetor for
the 1977 Super Six engine, the two-barrel Car-
ter for the 318 also has some engineering fea-
tures you should know about in order to service
them properly.
THE THROTTLE DUMP VALVE
This wide-open throttle dump valve was first in-
troduced about the middle of March, 1976 on
some selected 318 engines. lts purpose is to
‘dump’ the carburetor venturi signal to the
vacuum amplifier by bleeding off the venturi
signal to atmosphere. When the venturi vac-
uum signal to the amplifier is “killed” the
Exhaust Gas Recirculation valve closes (and
cannot open). With the EGR valve closed, there
IS no dilution of the fuel/air mixture with
exhaust gases during wide-open throttle driv-
ing conditions.
DUMP VALVE ADJUSTMENT
If the dump valve had to be removed from its
bracket or you suspect that it is not set proper-
ly, it's a simple job to check the adjustment.
First, move the throttle valves so they are wide
open and hold this throttle position. Next, place
LED 7
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BN
Wy TY
5
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Fig. 33 — Checking clearance of dump valve
a .060" round feeler gauge between the valve
operating lever and the plastic face of the valve
body. There should be a slight drag on the
gauge. If the gauge is too tight or too loose,
loosen the locknut, then turn the dump valve
body inward or outward until the measurement
Is correct. Tighten the lock nut. No other ser-
vice to this unit is required or necessary.
HIGH-ALTITUDE CALIBRATION
Some truck models with the 318 engine that
are sold for principal usage in high-altitude
areas of the country (altitudes of 4000 feet or
more above sea level) have an idle and main
fuel system compensation device. Basically, it is
a manually adjusted ''cap” that is part of the
venturi cluster assembly. It is spring loaded and
adjustable by means of a small screw extending
through the center section.
ADJUSTABLE
CLUSTER |
| A |
a LI
"x CUA —
Fig. 34 — Manually adjusted high altitude compensator
PURPOSE CF HIGH-ALTITUDE CALIBRATION
Engines equipped with the high-altitude cali-
bration carburetor are shipped from the factory
with the cluster ‘cap’ pulled down snug by
means of the adjusting screw. As a result, the
auxiliary idle air bleeds are blocked and the en-
gine receives sea level fuel/air mixtures.
a ADJUSTING
Fig. 35 — Adjusting screw is spring-loaded
ADJUSTMENT
During pre-delivery of the vehicle, the small
adjusting screw should be checked to first
make sure it is turned down (clockwise) snug
BUT NOT OVERLY TIGHT. Then back-off on the
adjusting screw at least one-and-one-half turns
(1-1/2) but not more than two complete turns.
The small spring will force the cap upwards.
As a result, the auxiliary bleeds are now opened
and auxiliary air can enter the idle system. Idle
and off-idle mixtures lean-out to the altitude
calibration required.
— =
= |
1 eu
= ei
SEA
LEYEL
SETTING -
7
Fig. 36 — Adjustable cluster cap’ seated
©
Ha
Besides the auxiliary air bleed there is an over-
size main air bleed drilled into the lower section
of the venturi cluster assembly. At sea level,
when the ‘cap’ is down snug against the
venturi cluster, the oversize main air bleeds are
covered. When the ‘cap’ is adjusted (backed-
off 1-1/2 to 2 turns) for high-altitude vehicle op-
eration, the main air bleeds are also uncovered
along with the auxiliary idle air bleeds. Inciden-
tally, the main air bleeds have larger drilled
passages than the auxiliary air bleeds.
HIGH
ALTITUDE
SETTING
Fig, 37 — Adjusting screw backed-off two turns
Fig. 38 — Checking dashpot adjustment
THE DASHPOT
When a 1977 engine is mated to a manual
transmission, the Carter two-barrel carburetor
is equipped with a mechanical dashpot, This
device simply “slows down’ the rate of throttle
closing in order to help prevent engine stall-out
when the accelerator pedal is released quickly
during rapid stops. Controlled air leakage
inside the unit is timed to permit gradual
retraction of the stem and diaphragm when the
throttle valves are released. When the throttles
are opened, the stem moves outward away
from the metal shell. When the throttle valves
close, an actuating tab on the throttle lever
presses against the stem and forces it to re-
tract progressively, balancing throttle return
spring pull against controlled air leakage.
DASHPOT ADJUSTMENT
To make a dashpot adjustment, the engine
must be at normal operating temperature and
curb idle speed and mixture set to the speci-
fications listed on the underhood emission
label.
Then, with a tachometer installed, start the
engine and open the throttle valves just
enough to allow the dashpot stem to become
fully extended.
Then slowly release the throttle lever until the
actuating tab on the lever contacts the stem of
the dashpot . . . BUT NOT depressing it. If the
dashpot is correctly set, the tachometer should
read 2500 RPM after engine speed stabilizes.
If the RPM is above or below specs, loosen the
lock nut on the bracket and turn the dashpot in
or out as necessary. Once the setting is correct,
then open the throttle and release it to make
sure the engine returns to the curb idle speed
called for on the emission label.
©
NAME
TEST
QUESTIONS
. The idle speed mixture restrictors located in
the throttle body just above the mixture
screws have been added to the 1977 Carter
two-barrel carburetors so that:
A. Inward movement of the idle mixture
screws is restricted.
В. Idle speed mixtures are prevented from
becoming excessively lean during idle
mixture adjustment.
C. Idle speed mixtures are prevented from
becoming excessively rich during idle
mixture adjustment.
The two notches cast into the underside sec-
tion of the carburetor flange on the 1977 Car-
ter two-barrel carburetor are needed to:
A. Supply heated air from the exhaust mani-
fold to warm the carburetor base.
B. Supply a source for a vacuum signal to the
PCV system.
C. Supply a vacuum source for the vacuum
kick diaphragm and the inlet air door
diaphragm located on the air cleaner as-
sembly.
. If you compare the “free travel” movement of
the vacuum step-up piston on 1977 Super Six
carburetors with that of the 1977 Carter two-
barrel carburetor on the 318 engine, you will
find:
A. The same amount of "free travel" on both
Carter two-barrel carburetor models.
B. There's more “free travel” movement onthe
Super Six two-barrel carburetor than there
is on the Carter two-barrel carburetor used
with the 318 engines.
C. There's a restricted amount of “free travel”
of the vacuum step-up piston on Super Six
carburetors as compared to that of the
two-barrel Carter used on 318 engines.
. Because of the greater amount of choke
offset on the Carter two-barrel carburetor for
the 1977 Super Six, air velocity rushing into
the intake manifold helps to keep the choke
closed during engine warm-up.
TRUE [] FALSE []
. When making the fast idle cam position
check, be sure the fast idle screw remains on
the high step of the cam.
TRUE [] FALSE []
6. When you check the setting of the vacuum
10.
step-up piston on the Carter two-barrel car-
buretor on the Super Six, always make sure
the tip of the idle speed screw is backed off
far enough so as not to contact the idle stop.
TRUE [] FALSE []
You should make the check of the accelerat-
ing pump stroke with the throttle valves com-
pletely closed.
TRUE [] FALSE []
. When making an on-the-car "wet" check of
float height, make sure extra fuel enters the
fuel bowl by pressing down on the floats.
Doing this ensures that the needle valve is
properly seated.
TRUE [] FALSE []
. Let's say that the fuel inlet fitting is somewhat
loose. By tightening it, the float needle also
moves inward. However, the float level does
not change.
TRUE [] FALSE []
If the float level is set too low, the level of fuel
in the main discharge nozzles will also be too
low causing excessively lean mixtures.
TRUE [| FALSE []
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