CARBURETOR
CARBURETOR CONSTRUCTION
FUEL AND LUBRICATION SYSTEM
4-2
(D Carburetor assembly No.
(2 Carburetor assembly No.
(8) Carburetor assembly No.
@ Carburetor assembly No.
® Plate
(6) Connector
@ E-ring
Jet needle
Throttle valve
Needle jet
AD Starter plunger
Spring
a3 Needle valve
Filter
d9 Pilot jet
E WN—-
Main jet
(D Main jet holder
Pilot screw
Float
29 Float arm pin
QD Carburetor top cap
22 Gasket
@3 Throttle lever assembly
29 Accelerator lever
(25) Spring
Accelerator rod
QD Accelerator plunger
Spring
@9 Throttle shaft assembly
Seal ring
BD Fuel drain screw
@2 Starter shaft
83 Throttle lever
Washer
69 Throttle stop screw
Throttle shaft lever
67 Throttle valve return spring
Steel ball
Spring
Screw
Air vent hose
42 Over flow hose
4-3 FUEL AND LUBRICATION SYSTEM
SPECIFICATION
ITEM SPECIFICATION
Type MIKUNI VM29SS
|. О. Мо. 27A01
Bore 29 mm
Idle r/min 1100 += 100 r/min.
Fuel level 2.0 + 0.5 mm
Float height 14.2 + 1.0 mm
Main jet (M.J.) #97.5
Main air jet (M.A.J.) 0.5 mm
Jet needle (J.N.) 6DP2-3rd
Cut-away (C.A.) 1.5 mm
Needle jet (N.J.) P-5
Pilot jet (P.J.) # 32.5
By-pass (B.P.) 1.0 mm
Pilot outlet (P.O.) 0.6 mm
Valve seat (V.S.) 2.0 mm
Starter jet (G.S.) #42.5
Pilot screw (P.S.) PRE-SET (% turn back)
Pilot air jet (P.A.J.) 1.6 mm
Throttle cable play 2.0 — 3.0 mm
1.D. NO. LOCATION
Each carburetor has 1.D. Number (1) stamped on
the carburetor body according to its specifications.
FUEL AND LUBRICATION SYSTEM +44
SLOW SYSTEM
This system supplies fuel during engine operation with throttle valve closed or slightly opened. The fuel
metered by the pilot jet is mixed with the proper amount of air metered by pilot air jet. This mixture, rich
with fuel, then goes up through pilot pipe to pilot screw. A part of the mixture is discharged into main bore
out of by-pass port. The remainder is then metered by pilot screw and sprayed out into the main bore
through pilot outlet.
7 =
Pilot screw f ||
Fuel/Air mixture
<<
Pilot jet
4-5 FUEL AND LUBRICATION SYSTEM
MAIN SYSTEM
This system supplies mixture during engine operation when the throttle valve is 1/4-Full open. The fuel in
float chamber is metered by main jet, and the metered fuel enters needle jet, in which it mixes with air
admitted through main air jet to form an emulsion.
The emulsified fuel then passes through the clearance between needle jet and jet needle and is discharged
into the venturi, where it meets main air stream being drawn by the engine.
Mixture proportioning is accomplished in needle jet; the clearance through which the emulsified fuel must
flow is large or small, depending ultimately on throttle valve position.
Main air jet
Fuel/Air mixture
—=
Main jet
FUEL AND LUBRICATION SYSTEM 4-6
STARTER SYSTEM
When the starter plunger is slided, the fuel metered by the starter jet is mixed with air coming from the
starter air passage. | |
The successive mixing of fuel with air is such that a proper fuel/air mixture for starting is produced when the
mixture is sprayed out through starter outlet into the main bore.
FLOAT SYSTEM
Floats and needle valve are associated with the _
same mechanism, so that, as the floats move up (
and down, the needle valve also moves likewise -
when fuel level is up in float chamber, floats are |
up and needle valve remains pushed up against
valve seat. Under this condition, no fuel enters J
the float chamber.
As the fuel level falls, floats go down and needle Y
valve unseat itself to admit fuel into the chamber. —
In this manner, needle valve admits and shuts off \
fuel alternately to maintain a practically constant
fuel level inside the float chamber.
Liss xxl
IC) o +
Do =
Needle valve
«фи Fuel
4-7 FUEL AND LUBRICATION SYSTEM
ACCELERATOR PUMP
When the throttle grip is suddenly and sharply opened, an excessive amount of air is intaken and the fuel/air
mixture will transiently become lean or thin. The function of the acceleration pump is to provide an ad-
ditional fuel flow in order to compensate for the lean fuel/air mixture, thereby attaining a proper fuel/air
mixture and improving the engine response.
As illustrated below, when the throttle valve is suddenly opened, the cam of the throttle valve shaft push
down the push rod via the pump lever. Consequently, the fuel in the pump chamber is fed through the
outlet passage and the outlet check valve (by pushing it up) to the accelerator pump nozzle which injects
the fuel into the main bore.
The outlet passage of the No. 1 and No. 3 carburetors is connected to the outlet passages of the other car-
buretors with rubber pipes.
When the throttle grip is closed, the plunger return spring pushes up the plunger, the fuel of the float chamber
is filled with fuel. |
Throttle lever
Pump lever
Push rod
Accelerator pump
nozzle
Plunger
FUEL AND LUBRICATION SYSTEM 48
DISASSEMBLY
e Remove the carburetor set lower plate (1)
by unscrewing eight bolts.
e Remove the carburetor set upper plate (2)
by unscrewing eight bolts.
e Loosen the four screws (3) holding the starter
shaft (4).
e Pull out the starter knob ©.
4-9 FUEL AND LUBRICATION SYSTEM
e Take out the spring (1) and steel ball (2) from
the No. 1 and No. 3 carburetors.
e Disconnect the three hoses (fuel (1), air vent
(2 and overflow (3).
e Remove the four top caps by unscrewing eight
SCrews.
e Remove the three screws and one bolt (1).
FUEL AND LUBRICATION SYSTEM 410
e Remove a screw (1) holding the throttle shaft.
e Separate the four carburetors by disconnecting
the hoses.
e Pull out the throttle valve shaft from the No. 3
carburetor.
e Remove the accelerator pump lever (1) by un-
screwing the bolt (2).
4-11 FUEL AND LUBRICATION SYSTEM
(M) Accelerator pump lever
(9) Spring
(3) Bolt
e Take out the throttle valve assembly and the
plastic washer (1).
e Remove the two allen screws 2).
e Separate the throttle valve related parts.
(D Throttle valve holder
@ Allen screw
® Throttle valve
@ Jet needle
FUEL AND LUBRICATION SYSTEM 4-12
e Remove the throttle lever bracket by unscrewing
the screw (1).
e Remove the float chamber body by unscrewing
four screws.
e Separate the accelerator parts from the float
chamber body.
(1) Float chamber body
@ Spring
(3 Plunger
(4) Rod
e Pull out the float pin 1) and remove the float.
CAUTION:
When removing the float pin, be careful not
to damage the carburetor body.
4-13 FUEL AND LUBRICATION SYSTEM
e Remove the needle valve (1), main jet (2) and
pilot jet 9).
e Check following items for any damage or
clogging.
* Pilot jet
Main jet
Main air jet
* Pilot air jet
Needle jet air bleeding holes
* Float
Needle valve mesh and O-ring
* Gasket and O-ring
Pilot outlet and bypass hole
NEEDLE VALVE INSPECTION
If foreign matter is caught between the valve seat
and the needle, the gasoline will continue flowing
and cause it to overflow. If the seat and needle are
worn beyond the permissible limits, similar trouble
will occur. Conversely, if the needle sticks, the
gasoline will not flow into the float chamber.
Clean the float chamber and float parts with
gasoline. If the needle is worn as shown in the
Illustration, replace it together with a valve seat.
Clean the fuel passage of the mixing chamber with
compressed air.
FLOAT HEIGHT ADJUSTMENT
To check the float height, invert the carburetor
body, with the float arm kept free, measure the
height (A) while float arm is just in contact with
needie valve by using calipers.
Bend the tongue (1) as necessary to bring the
height (A) to this value.
Fuel pipe O-rings
Accelerator pump
NOTE:
Identity the four throttle valves removed as
No. 1 through No. 4, in order to make sure
each will be restored to the carburetor from
which it was taken out.
Float height ® 14.2 + 1.0 mm
09900-20102 Vernier calipers
FUEL AND LUBRICATION SYSTEM 414
NOTE:
Be sure not to compress the spring in the
needle valve.
REASSEMBLY
e Align the groove (1) of the needle jet with the
pin 2).
e When installing the throttle lever bracket, align
the hole (1) and apply thread lock cement to
the screw (2).
e When inserting the throttle shaft to the throttle
valve bracket of the No. 3 carburetor, be sure to
install the plastic washer (1).
4-15 FUEL AND LUBRICATION SYSTEM
® Correctly install the accelerator pump lever (1)
and spring (2) for the No. 1 and No. 3 carbu-
retor.
e Correctly install the throttle lever return spring
for the No. 3 carburetor.
e Correctly position the accelerator pump lever (1)
on the cam (2) of the throttle lever for the No. 1
and No. 3 carburetor.
e When engaging four carburetors, be sure to fix
fuel pipe (1) with clips, air vent connector 2)
with O-rings ® and accelerator pipe (4) with
clips properly.
_
FUEL AND LUBRICATION SYSTEM 416
e Apply thread lock” 1342"to the upper and
lower plate screws.
99000-32050 Thread lock"1342”
After all work is completed, mount the carbu-
retors on the engine and the following ad-
justments are necessary.
Engine idle r/min .................Page 2-10
Throttle cable play ................Page 2-10
Balancing carburetors . ..…........ Page 4-17
FUEL LEVEL INSPECTION
e Remove carburetor drain plug (1) and install
the fuel level gauge (2).
09913-14511 Fuel level gauge
e Run the engine at the idling speed (1 000—1 200
r/min), and measure the distance (A) with the
middle line of the level gauge aligned with the
mating surface of float bowl as shown in photo.
(A) should be within the specified range.
Distance (A) 2.0+0.5mm
4-17 FUEL AND LUBRICATION SYSTEM
BALANCING CARBURETORS
Check the four carburetors for balancing according
to the following procedures.
As the first step, calibrate the carburetor balancer
gauge as follows.
09913-13121 Carburetor balancer
09913-12710 Adaptor
e Start up the engine and run it in idling condition
for warming up.
e Stop the warmed-up engine. Remove the
vacume inlet screw (1) for No. 1 or No. 4
cylinder and install adaptor (2) with O-ring.
e Connect one of the four rubber hoses of the
balancer gauge to this adaptor, and start up the
engine, and keep it running at 1750 r/min by
turning throttle stop screw (3).
e Turn the air screw (4) of the gauge so that the
vacuum acting on the tube of that hose will
bring the steel ball (5) in the tube to the center
line ©.
After making sure that the steel ball stays steady
at the center line, disconnect the hose from the
adaptor and connect the next hose to the
adaptor. Turn air screw to bring the other steel
ball (1) to the center line.
Repeat the process on the third and forth tubes.
The balancer gauge is now ready for use in
balancing the carburetors.
Remove the four carburetor top caps by un-
screwing respective screws.
Remove the respective vacuum inlet screws and
insert the adaptor in the holes. Connect the
balancer gauge hoses to these adaptors, and balance
the four carburetors as follows.
Start up the engine, and keep it running at
1750 r/min.
A correctly adjusted carburetor has the steel
balls in the Nos. 1 through 4 tubes at the same
level, as shown,
If the steel balls are not in correct positions,
loosen the lock nut and adjust the throttle
valve adjusting screw correctly. After adjusting,
tighten the lock nut securely.
Adjusting order:
FUEL AND LUBRICATION SYSTEM
4-18
No. 2 carb. > No. 1 carb. = No. 4 carb.
NOTE:
When removing the vaccum inlet screws, be
carefull not to drop them.
4-19 FUEL AND LUBRICATION SYSTEM
e After balancing the carburetors, set its speed
between 1 000 and 1 200 r/min by turning the
throttle stop screw (1.
CAUTION:
Do not disturb the pilot screw. This com-
ponent is pre-set at the factory by the very
specialized equipment.
NOTE:
* If an adjustment is required, it is suggested
that the fuel tank is removed, and fuel
should be supplied by a separate fuel tank.
LUBRICATION SYSTEM
OIL PRESSURE
Start the engine and check if the oil pressure
indicator light is turned on. If it keeps on lighting,
check the oil pressure indicator light circuit.
If it is in good condition, check the oil pump
pressure in the following manner:
e Install the oil pressure gauge in the position
shown in the figure: (at the oil pan)
e Warm up the engine as follows:
Summer 10 min. or so at 2000 r/min
Winter 20 min. or so at 2000 r/min
e After warming up operation, increase the engine
speed to 3000 r/min, and read the oil pressure
gauge.
Oil pressure specification
Above 300 kPa (3.0 kg/cm?)
Below 600 kPa (6.0 kg/cm?)
at 3000 r/min. Oil temp. at 60°C (140° F)
09915-74510 Oil pressure gauge
NOTE:
Engine oil must be warmed up to 60°C
(140° F) when checking the oil pressure.
If checking the oil pressure at the main gallery of
the engine right side, use the adaptor.
09915-74540 Adaptor
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