P/N 0-80674 (650 cfm), 0-80675 (750 cfm), 0-80676 (950 cfm)
Installation and Adjustment Instructions
CONGRATULATIONS on your purchase of Holley ULTRA HP Series carburetor! We feel that you have purchased the finest
performance carburetor manufactured today. Should you need information or parts assistance, please contact our Technical
Service Department at 1-866-GOHOLLEY or 1-270-781-9741, Monday through Friday, 7 a.m. to 5 p.m. CST. Please have the
part number of the product you purchased on hand when you call.
WARNING! These instructions must be read and fully understood before beginning installation. Failure to follow
these instructions may result in poor performance, vehicle damage, personal injury, or death. If these
instructions are not fully understood, installation should not be attempted.
Double -step Down Leg
Double -step Down Leg
NOTE: These carburetors are not legal for use in California on any pollution-controlled motor vehicle.
Holley Performance Products has written this instruction sheet for the installation of the Model 4150 ULTRA HP carburetors.
This instruction sheet contains all the information needed to install these carburetors. Please read all the WARNINGS and
NOTES, as they contain valuable information that can save you time and money. Holley Performance Products cannot and will
not be responsible for any alleged or actual engine or other damage, or other conditions resulting from misapplication of the
carburetor described herein. However, it is our intent to provide the best possible products for our customer; products that
perform properly and satisfy your expectations. Should you need information or parts assistance, please contact our Technical
Service Department at 1-270-781-9741, Monday through Friday, 7 a.m. to 5 p.m. Central Time; please have the part number of
the product that you purchased on hand when you call.
This instruction sheet cannot contain all of the information, which may be desired by some individuals. Further clarification is
available in HOLLEY CARBURETORS (P/N 36-73), published by H.P. books and available at your Holley distributor.
An in-line fuel filter should be installed between the fuel pump and the carburetor.
Recommended fuel pressure should be set at 7-1/2 psi maximum; 5 psi minimum. Fuel pressures above 7-1/2 psi can
create severe fuel control problems and are not recommended.
Fuel lines should be a minimum of 3/8”.
A non-restrictive open-element air cleaner kit is recommended (Holley P/N 120-102).
The pump cam adjustment kit is available under Holley P/N 20-12.
Slosh tubes for preventing fuel starvation during rapid acceleration are available under Holley P/N 122-5000.
WARNING! Prior to and after installing your new carburetor, manually operate the throttle lever, checking for any
sticking or binding. Failure to do so may result in a runaway engine or a wide-open throttle condition,
which could result in engine damage, personal injury, and/or death.
NOTE: Inspect the carburetor, looking for any missing parts, bent levers, or any possible shipping damage.
Remove the air cleaner.
Label any connections to the carburetor, such as the fuel line.
After labeling all connections to the carburetor, carefully disconnect all hoses and lines. When removing the fuel line, slide a
rubber cap plug over the end to prevent fuel from running out, which may create a fire hazard. Use a clean metal container
to collect any spilled fuel.
Remove the throttle linkage and automatic transmission controls from the throttle lever. Disassemble and save the throttle
return spring.
Remove the two front and two rear-attaching manifold flange nuts.
Remove the throttle cable bracket, if so equipped (located at the right rear-attaching bolt).
Remove the carburetor by lifting it straight upward. Sometimes, the carburetor can stick to the manifold gasket, requiring it
to be pried loose. Before prying, double check to make sure all the carburetor attaching bolts and connections have been
Place clean shop towels or rags into the manifold opening to prevent dirt or debris from entering the engine. Keep exposed
ends of vacuum and fuel lines free from dirt.
WARNING! Failure to cover the intake opening with a clean towel could result in dirt or debris entering the engine.
Dirt or debris in the induction system can cause engine damage, which may necessitate a complete
engine overhaul.
Remove the gasket from the intake. Remove any gasket material that may have adhered to the manifold. DO NOT gouge
the intake manifold sealing surface during removal of old gasket material.
10. Remove the shop towels from the intake and vacuum out the intake channel to ensure no dirt or debris is left in the intake
system. Place a clean shop towel over the entire intake opening until you are ready to install the new carburetor.
During fuel line installation, be careful to avoid introducing any dirt particles that could enter the fuel inlet and jam open the
needle and seat resulting in carburetor flooding, malfunctioning, and/or possible engine fire. In all cases where the fuel line has
been cut, it is essential that it be cleaned to prevent contamination from entering your new carburetor. The fuel line must be
flushed of rust, dirt, and other debris.
DANGER! Holley DOES NOT recommend the procedure where the coil wire is disconnected, the engine cranked for a
few revolutions, and the fuel is collected in a container. This procedure is unsafe, because sparking can
occur either at the coil, or at the distributor end of the coil wire, which may ignite any fuel spilled in the
engine compartment. This could cause an EXPLOSION or FIRE, which may result in serious injury or
WARNING! Flush fuel lines only in a well-ventilated area and away from all sources of heat or flame. Failure to follow
these instructions may cause gasoline vapors to ignite resulting in a fire or explosion, which may result in
serious injury or death.
WARNING! Wear eye protection when performing this step. Failure to wear eye protection can result in gasoline or
other contaminates entering the eye, which could result in permanent eye damage or blindness.
Disconnect the fuel line at the fuel pump.
Using a compressed air source, blow the fuel line clean.
WARNING! Holley Performance Products highly recommends that a quality fuel filter be installed with any
replacement carburetor to catch any dirt that may still remain in the system. Any dirt that may enter the
carburetor can cause the carburetor to flood or malfunction. A carburetor that has a malfunction caused
by dirt in the system due to negligence of the owner will void the warranty.
Install a flange gasket on the manifold. If a spacer is being used, the installation order will be: flange gasket, spacer, and
another flange gasket over the manifold stud bolts.
WARNING! The carburetor should be installed directly onto its manifold without an adapter whenever possible.
Sometimes an adapter can create problems with hood clearance, airflow, throttle linkage, fuel line
attachment, and/or fuel mixture distribution.
Place the carburetor in position over the four stud bolts and secure in place. To avoid warping the carburetor flange, tighten
each bolt gradually. Tighten in a criss-cross pattern to 60 in./lbs. Be careful not to over-tighten the nuts.
WARNING! Over-tightening the carburetor manifold flange hold-down nuts may result in a warped or cracked
carburetor throttle body. The carburetor hold-down nuts should be tightened down progressively in a
criss-cross pattern to 60 in./lbs., to prevent vacuum leaks and to avoid causing damage to the throttle
body. A carburetor that has been damaged due to negligence of the owner will void the warranty.
Connect the fuel lines, throttle linkage, and return springs. Operate the linkage to ensure correct travel by fully opening and
closing the throttle by hand.
Without operating the throttle, crank the engine. It may take 15 to 30 seconds of cranking to allow the fuel bowls of the
carburetor to fill. If the engine does not start, stop cranking, open and close the throttle twice and crank again until the
engine starts.
WARNING! DO NOT crank the engine for more than 15 seconds at a time. Cranking longer than 15 seconds can
overheat the starter, resulting in premature starter failure.
After starting the engine, check the fuel lines and inlet fittings for possible fuel leaks.
WARNING! If any fuel leakage or weeping is detected, shut off the engine immediately, and wipe up any fuel. Locate
the source of the leak and correct before proceeding any further.
Before you begin to tune your carburetor for your particular vehicle, you must get a “feel” for your vehicle’s performance, so that
any changes you make (Good or Bad) will be readily apparent. Be patient and make only one change at a time, so that only
that change can be fully analyzed. This cannot be overemphasized, as there are no “short-cuts” to peak performance.
Recording each change and the resulting performance increase or decrease will provide you with a “Handbook” of how vehicle
performance is affected by individual carburetor adjustments. This may be helpful in the future or on other applications.
The float(s) controls the fuel delivery, however if the float(s) are not properly adjusted a fuel starvation or a flooding effect could
result. This operation is difficult to do accurately on a rough-idling vehicle.
Figure 1
For the mechanical fuel pump, remove the coil wire and crank the engine over for 10 seconds to allow fuel bowls to fill. This
procedure can prevent a power valve blow out. Reconnect the coil wire when finished. Electric fuel pumps let the fuel
bowls fill in stages by turning the ignition on and then off. Let the fuel pump run for a few seconds at a time. This procedure
can prevent the needle from being forced up at an angle (not allowing the needle to seat properly).
Start the vehicle.
The fuel level in the clear sight glass should be mid-level to slightly below center for correct float adjustment. Most likely,
this will not need to be adjusted. See Figure 1.
If you do need to adjust this, loosen the lock screw and turn the adjusting nut clockwise to lower the fuel level and
counter-clockwise to raise the fuel level.
Tighten the lock screw while holding the adjustment nut. Make sure the fuel level is mid-level to slightly below center of the
sight glass.
If necessary, flush the bowl by revving the engine a few times with the transmission in neutral.
The idle speed screw in most cases is the only screw you should adjust. The idle speed screw controls the throttle plate position
at idle, which in turn raises or lowers the engine rpm by allowing more or less air/fuel mixture into the engine. It does not control
the air/fuel mixture. Here are the proper steps for setting the engines idle speed.
Find the proper idle rpm on the underhood decal of your vehicle. If this decal is not available, find a service manual that
references your vehicle and engine to find the recommended idle rpm.
Start the engine and allow it to warm up.
Connect a tachometer, if your vehicle is not equipped with one.
Make sure the parking brake is on and the wheels are blocked. Place the automatic transmission in drive, the manual
transmission in neutral, or as stated on the underhood decal.
If the idle speed is slower than recommended, turn the screw clockwise to speed up the rpm. If the idle speed is too fast,
turn the idle screw counter-clockwise to lower the rpm. This adjustment should be made to both the primary and secondary
screws in equal amounts, so that the throttle plates are opened the same amount.
NOTE: The Holley 4150 ULTRA HP includes the secondary idle adjustment lever, P/N 26-137, that allows the
secondary throttle plate to be adjusted without carburetor removal.
Figure 2
Figure 3
Idle mixture screws control the air/fuel mixture at idle. The amount of air/fuel mixture used at idle is controlled by engine
vacuum. So, when tuning the idle mixture, you are actually tuning for the best manifold vacuum. Idle mixture screws are found
on the metering blocks. Your carburetor will have four idle mixture screws; one for each venturi. This is known as four-corner
idle. If you change one idle mixture screw, you are required to change the other idle mixture screws the same amount. Here are
the proper steps for setting the idle mixture screws.
Figure 4
Attach the vacuum gauge to a manifold vacuum source.
Adjust each idle mixture screw the same amount to achieve the highest possible vacuum reading without increasing the idle
speed screw.
Now that the idle mixture is set, it may be necessary to go back and reset the idle speed using the idle speed screw.
Continue back and forth between the tuning of the idle mixture screws and idle speed screws, until little change is noticed in
manifold vacuum and idle speed is correct.
The accelerator pump’s purpose is to make up for the lag in fuel delivery. This enables the engine speed to increase in
response to throttle opening. Differences in vehicle weight, transmissions, and rear axle ratios affect the amount of fuel and the
delivery rate that should be provided by the accelerator pump. This may necessitate the customizing of your accelerator pump
to your vehicle and its use.
NOTE: The old saying, “if a little is good, a lot is better”, does not apply to the proper tuning of the accelerator pump.
Your car’s performance can be just as bad if it receives “too much fuel too soon”, as if it receives “too little fuel
too late.”
Two factors that affect the accelerator pump’s delivery are the pump cam and the pump shooter (discharge nozzle). The pump
cam determines the total volume of fuel and affects delivery rate; the pump shooter affects delivery rate and helps determine the
duration of the shot.
In general, the #1 locations on the pump cams provide a moderate initial delivery and have a greater final delivered volume. The
#2 locations on the pump cams provide a greater initial delivery and have a lesser total delivered volume. The pump shooters
have a number stamped on their casting, which designates the shooter size in thousandths of an inch, i.e., a #25 shooter has a
.025” discharge orifice. The smaller diameter nozzles lengthen the pump shot duration and are used with heavier vehicles or
with vehicles equipped with lower numerical rear axle ratios. Larger diameter nozzles (.035 - .037) shorten the pump shot
duration, but deliver a greater initial volume of fuel. These sizes should be used on applications where engine speed will
increase rapidly (vehicles with good power-to-weight ratios). Best acceleration is achieved when the accelerator pump delivers
the lean, best-power, air/fuel ratio to the engine; not when the maximum volume of fuel is supplied.
An important point should be kept in mind when tuning a Double Pumper, the secondary accelerator pump must supply fuel for a
sufficient time, so that the secondary main nozzles can “start up” and deliver fuel to the engine after the secondary throttle plates
are opened. If the nozzles do not start by the time the pump shot expires, bogging will result. To apply the information above,
follow these steps for tuning the accelerator pump.
Best starts or racing starts usually result when the rear tires break loose for the first 10 to 25 feet. This allows engine speed to
increase rapidly to get the secondary main nozzles started. If wide tires, which provide extremely good traction, do not permit
the rear tires to break loose, bogging can result. Best times may be recorded with narrower tires.
Figure 5
Figure 7
Figure 6
Figure 8
Figure 9
Change pump shooters, until the smallest diameter nozzle that provides the crispest response is found.
Then change the pump cams and locations until the right cam is found that provides even more response.
Finally, change the pump shooter once again, until the crisp response is maximized.
NOTE: If a nozzle size is desired that seems “in between” the nozzle sizes provided, then the nozzle can be drilled to
the desired size by using a wire drill held in a pin vise.
At this point, there should be no bogs, flat spots or black smoke (indicating excessive richness) when accelerating at wide
open throttle from a standing start.
Due to varied applications that a universal performance carburetor will work with, no additional tuning jets have been included.
However, a few tips on jetting are provided to help you understand their purpose. Holley’s Quick Change Fuel Bowls are
recommended if repeated changes or experimentation with the main jets will be performed.
Out of the box jetting is extremely close for most applications.
In most cases, it will be unnecessary to increase jet size more than four numbers greater than out of the box jetting.
However, exceptions could arise when the carburetor is mounted on a very large volume, plenum-ram manifold.
Carburetors are calibrated at 70° at sea level. Decrease the jet size one number (approx. .002) for approximately every
2000 ft. increase in altitude. Increase jet size one number for every 35° drop in temperature.
Holley jets are broached, flowed and stamped according to flow rate. Never drill jets, this seriously alters flow
characteristics. Stamped numbers are reference flow numbers and DO NOT indicate drill size.
Spark plugs provide the best indication of proper jetting. Allow plugs to cool before jumping to conclusions.
IMPORTANT! It is recommended that you document the existing adjustable bleed sizes that make up this metering
block BEFORE any changes are made. This billet adjustable metering block allows the user to tune the
intermediate circuit restrictions, idle feed restrictions, emulsion bleed restrictions, power valve, and
main jets.
IMPORTANT! Holley highly recommends that only an experienced carburetor tuner [with access to a dynamometer
that monitors air/fuel ratio and BSFC (Brake Specific Fuel Consumption)] make any changes to the
factory restrictor settings, as received from Holley. Furthermore, Holley is not responsible for any
engine damage that may occur due to the carburetor being changed from the original factory calibration.
Power Valve Channel Restrictions (PVCR):
These two restrictions are visible when the power valve is removed. They meter the flow of fuel into the main well. It is the
PVCR diameter, not the openings in the power valve, which controls the amount of fuel admitted in the circuit. Changing the
size of these restrictions will lean/enrich the air/fuel ratio at full power, but will have no effect at idle or during part throttle
operation, when the power valve is closed.
Idle Feed Restrictions (IFR):
The idle circuit calibration is determined by the diameter of the IFR and the idle air bleed. An IFR is nothing more than a
metering jet for the idle system and the air bleed serves as an air/flow-regulating orifice. Turning the idle mixture screw varies
the volume of air/fuel emulsion discharged into the manifold, not the ratio of air to fuel in the emulsion. Typically, as camshaft
duration and overlap are increased, idle fuel jetting (idle feed restriction size) must be correspondingly increased to compensate
for dilution and scavenging (where an amount of intake charge is drawn out through a late closing exhaust valve).
2 Circuit Billet Metering Block (Front) Model 4150 Ultra HP Series
Figure 10
Emulsion Bleeds:
The emulsion bleeds are best tuned on the dynamometer. Bleed utilization (size and location) can vary, as will horsepower
gains, depending upon the engine. Not all of the emulsion bleed holes have to be used to obtain peak performance. Gains have
been documented on use of as little as three (one side of metering block) of the bleed holes.
NOTE: The emulsion bleed tuning kit (P/N 36-22) will support tuning of this metering block. It contains ten each of the following
bleed sizes: Blank (no hole), .020”, .031”, .040”, .046”, and .052”. Individual bleeds are available from blanks to .082”
from your Holley dealer under individual P/Ns 142-xx (xx=size).
WARNING! Adjustment of the air bleeds is not recommended. A competent mechanic with a complete and thorough
knowledge of carburetors, fuel systems, and engine requirements should only perform air bleed
adjustments. Failure to follow these recommendations may result in a lean fuel/air mixture causing severe
engine damage, property damage, serious injury, and/or death.
WARNING! Air bleed sizes should not be adjusted more than six (6) sizes in any one direction from the original air
bleeds, as shipped from Holley. Air bleed adjustment beyond six (6) sizes could result in a lean fuel/air
mixture causing severe engine damage, property damage, serious injury, and/or death.
Experimenting with air bleeds is not recommended and should only be attempted by an expert carb tuner. Countless hours of
testing have been performed on expensive flow stands to obtain the proper bleed size for a given calibration. It is unlikely that a
better air bleed calibration can be obtained, however the 4150 ULTRA HP Series are equipped with removable air bleeds. Here
is some basic knowledge of how air bleeds work.
The main or high-speed air bleeds affect the entire range of the main-metering system. The purpose of the main metering
system and main air bleeds is to emulsify the fuel before entering the discharge nozzle to be discharged into the air stream in the
venturi. The fuel/air mixture becomes leaner as air bleed size is increased. Decreasing the size of the main air bleeds will
decrease pressure across the main jet, which in turn will pull more fuel through the main system creating a richer fuel/air mixture.
The main or high speed air bleeds also act as an anti-siphon or siphon breaker, so fuel does not continue to discharge or dribble
into the venturi after airflow is reduced or stopped. At high speeds, the fuel/air mixture must be on the rich side to prevent
damage to the engine.
The idle system supplies fuel at idle and low speeds. The idle system requires a richer mixture than at cruise speed. Unless the
idle mixture is richer, a slow and irregular combustion will occur, known as a rough idle. Decreasing the idle air bleed size
richens the idle mixture by increasing the pressure drop in the system. Increasing idle air bleed size leans the idle mixture by
reducing the pressure drop across the idle air bleeds. The same conditions can be created by backing out the idle mixture
screws, which will increase the pressure across the idle air bleeds, pushing more fuel from the idle well creating a richer fuel/air
ratio. The idle mixture screw is the only adjustment recommended for controlling the idle fuel/air mixture richness or leanness.
NOTE: See Figure 11 for air bleed locations and identification. It is recommended that all jet sizes be documented
before any tuning of the air bleeds or main jets is started. Below is a chart for recording the jet and air bleed
sizes for your 4150 ULTRA HP Series carburetor, as shipped from Holley. Should you adjust the air bleed size
or main jet size, this chart will allow the tuner to return the carburetor to the original jetting. Please place this
information in a safe place along with any other documentation for your carburetor.
High Speed
High Speed
Figure 11
The number stamped on a power valve, such as 65, indicates the manifold vacuum below which the power valve is operational.
In this case, all manifold vacuums below 6.5” Hg., the power valve is operating. Generally, a 65 power valve is sufficient for
most high performance applications that have a manifold vacuum of 12” Hg or higher. However, some problems can result with
radically cammed, full-race machines equipped with automatic transmissions. These vehicles often “idle” at 2000 rpm, approx.
6.0” Hg. At this point, the main nozzles are starting to feed and richen the mixture (supplied by the power valve) and the engine
will probably “load up”. To correct this problem, install a 45 or 35 power valve. If the engine has a manifold vacuum of 12” Hg or
less, a good way to determine power valve size is take the manifold vacuum at idle and divide that number by two. The answer
is the power valve size. This will provide idling and proper fuel flow under wide-open throttle conditions when manifold vacuums
seldom rise above 1” Hg.
Slosh tubes can be used in the secondary jets to prevent the secondary main jets from being uncovered as fuel rushes to the
back of the fuel bowl during extreme accelerations.
Holley Performance Products
Toll Free Technical Service: 1-866-GOHOLLEY
Technical Service: 1-270-781-9741
Fax: 1-270-781-9772
For online help, please refer to the Technical Information section of our website:
Date: 2-5-04
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