Troubleshooting Your Holley Carb (2022)

Troubleshooting Your Holley Carb (1)

Troubleshooting Your Holley Carb (2)

Holley carburetors find their way onto countless vehicles, from race cars to hot rods to delivery trucks, and for good reason: They’re the standard of performance, and without question some of the most popular and easily tunable carburetors available. Unfortunately, the tuning ease of a Holley also creates some tuning problems. Because the carburetors are easy to tune, they’re also easy to screw up (especially when “tuned” by the wrong hands). Because of that, we’ve compiled the following back-to-basics look at troubleshooting Holley carbs. Check out the guide that follows. It’s applicable to all popular 4150, 4160 and 4500 series carburetors. The information included is something you’ll want to save. And by the way, there’s something here for everyone, from the novice to the seasoned veteran.


(information courtesy of Holley Performance Products)

In order for a carburetor to function correctly, it requires the following:

(1) Fuel Supply

(2) Linkage and emission control systems

(3) Engine Compression

(4) Ignition system firing voltage

(5) Secure intake manifold

(6) Engine temperature

(7) Carburetor adjustments

Any problems in the above areas can cause the following:

(1) No start or hard starting – hot or cold

(2) Rough engine idle and stalling

(3) Hesitation on acceleration

(Video) Fixing The Great Holley Carburetor Mystery Issue

(4) Loss of power on acceleration and top speed

(5) Engine to run uneven or surge

(6) Poor fuel economy

(7) Excessive emissions


Note: Be sure that all emission control units are installed and operating properly. This includes all emission system solenoids and connecting hoses.

Carburetor problems cannot be isolated effectively unless all other engine systems are operating correctly and the engine is properly tuned.

Troubleshooting Your Holley Carb (3)


Improper starting procedure usedCheck to determine if proper starting procedure is used, as outlined in owner’s manual
No fuel in gas tankAdd fuel. Check fuel gauge for proper operation.
Choke valve not closing sufficiently when coldAdjust the index of the choke thermostatic (bi-metal) coil
Choke valve or linkage binding or stickingRealign the choke valve or linkage as necessary. If caused by dirt and gum, clean with automatic choke cleaner. Do not oil choke linkage. If parts are replaced, readjust to specifications.
No fuel in carburetor(1) Remove fuel line at carburetor. Connect hose to fuel line and run into metal container. Remove the high tension wire from the center tower on distributor cap and ground. Crank over engine – if there is no fuel discharge from the fuel line, check for kinked or bent lines. Disconnect fuel line at tank and blow out with air hose, reconnect line and check again for fuel discharge. If none, replace fuel pump. Check pump for adequate flow, as outlined in factory service manual.

(2) If fuel supply is o.k., check the following:

(a) Inspect fuel filter(s). If plugged, replace.

(b) If filters are o.k., remove air horn or fuel bowl and check for a bind in the float mechanism or a sticking float needle. If o.k., adjust float as specified.

Engine flooded.

NOTE: to check for flooding remove the air cleaner. With the engine off look into the carburetor bores. Fuel will be dripping off nozzles and/or the carburetor will be very wet.

Be sure that the proper “unloading” procedure is being used. Depress the accelerator to the floor and check the carburetor to determine if the choke valve is opening. If not, adjust the throttle linkage and unloader.
Carburetor flooding.NOTE: Before removing the carburetor air horn, use the following procedure which may eliminate the flooding:

(1) Remove the fuel line at the carburetor and plug. Crank and run the engine until the fuel bowl runs dry. Turn off the engine and connect the fuel line. Then re-start and run engine. This will often flush dirt past the carburetor float needle and seat.

(2) If dirt is in the fuel system, clean the system and replace filter(s) as necessary. If excessive dirt is found, remove the carburetor unit. Disassemble and clean.

(3) Check float needle and seat for proper seal. If the needle is defective, replace with a Holley matched set.

(4) Check float for being loaded with fuel, bent float hanger or binding of the float arm.

(5) Adjust float to specifications.

(Video) Troubleshooting Holley Hesitation Off Idle

Troubleshooting Your Holley Carb (4)


Choke valve not opening completely.(1) Check for binding choke valve and/or linkage. Clean and free-up or replace parts as necessary. Do not oil choke linkage.

(2) Check and adjust choke thermostatic coil.

(3) Check for choke thermostatic coil binding in well or housing.

(4) Check for vacuum leak with integral choke system.

Engine flooded – Carburetor flooding.See procedure under “Engine cranks, will not start”
No fuel in carburetor.(1) Check fuel pump. Run pressure and volume test.

(2) Check float needle for sticking in seat, or binding float.

Leaking float bowl.Fill bowl with fuel and check for leaks.
Fuel percolation.Open throttle wide and operate starter to relieve over rich condition.
Troubleshooting Your Holley Carb (5)


Engine does not have enough fast idle speed when cold.Check and re-set the fast idle setting and fast idle cam.
Choke vacuum diaphragm unit is not adjusted to specifications or unit is defective.(1) Adjust vacuum break to specification.

(2) If adjusted O.K., check the vacuum opening operation as follows:

(a) On externally mounted vacuum diaphragm unit, connect a piece of hose to fitting on the vacuum diaphragm unit and apply suction preferably ∫y hand vacuum pump or another vehicle. Plunger should move inward and hold vacuum. If not, replace the unit.

(b) On the integral vacuum piston unit, remove cover and visually check piston and vacuum channel. If piston is sticking, replace assembly.

NOTE: Always check the fast idle cam adjustment before adjusting vacuum unit.

Choke coil rod out of adjustment.Adjust choke coil rod.
Choke valve and/or linkage sticking or binding.(1) Clean and align choke valve and linkage. Replace if necessary.

(2) Re-adjust if part replacement is necessary.

Idle speed setting.Adjust idle speed to specifications on decal in engine compartment.
Not enough fuel in carburetor.(1) Check fuel pump pressure and volume.

(2) Check for partially plugged fuel inlet filter. Replace if dirty.

(3) Remove air horn or fuel bowl and check float adjustments.

Carburetor flooding.(1) Check float needle and seat for proper seal. If needle is defective, replace with a Holley matched set.

(2) Check float for being loaded with fuel, bent float hanger or binding of the float arm.

(3) Check float adjustments.

(4) If excessive dirt is found in the carburetor, clean the fuel system and carburetor. Replace fuel filters as necessary.

Troubleshooting Your Holley Carb (6)


(Video) Holley Carb Flooding Out Vent Tube | Troubleshooting Holley Carburetors | Holley Carb Secrets |

Idle mixture adjustmentAdjust idle mixture screws to lean best idle. Repeat the operation on 2 and 4V carburetors. Now, turn mixture screws in until idle speed drops 25 RPM on tachometer.
Idle speed setting.Re-set idle speed per instructions on decal in engine compartment. Check solenoid operation.
Manifold vacuum hoses disconnected or improperly installed.Check all vacuum hoses leading to the manifold or carburetor base for leaks, being disconnected or connected improperly. Install or replace as necessary.
Carburetor loose on intake manifold.Torque carburetor to manifold bolts (100 inch pounds)
Intake manifold is loose or gaskets are defective.Using a pressure oil can, spray light oil or kerosene around manifold legs and carburetor base. If engine RPM changes, tighten or replace the manifold gaskets or carburetor base gaskets as necessary.
Hot idle compensator not operating (where used)Normally the hot idle compensator should be closed when engine is running cold and open when engine is hot (approximately 140° F at comp.) replace if defective.
Carburetor flooding

NOTE: Check by using procedure under “carburetor flooding”.

(1) Remove air horn and check float settings.

(2) Check float needle and seat for proper seal. If the needle is defective, replace with a Holley matched set.

(3) Check float for being loaded with fuel, bent float hanger or binding of the float arm. Adjust to specifications.

(4) If excessive dirt is found in the carburetor, clean the fuel system and carburetor. Replace fuel filters as necessary.

Troubleshooting Your Holley Carb (7)


Fuel restriction.Check all hoses and fuel lines for bends, kinks or leaks. Straighten and secure in position. Check all fuel filters. If plugged or dirty, replace.
Dirt or water in fuel system.Clean fuel tank and lines. Remove and clean carburetor.
Fuel level.Adjust float. Check for free float and float needle valve operation.
Main metering jet defective, loose or incorrect part.Replace as necessary.
Power system in carburetor not functioning properly.Power valve or piston sticking in down position. Free up or replace as necessary.

Power valve loose, incorrect gasket or leaking around threads. Replace as necessary.

Leaking diaphragm. Test with Holley hand vacuum pump. Replace as necessary.

Vacuum leaks.It is absolutely necessary that all vacuum hoses and gaskets are properly installed with no air leaks. The carburetor and manifold should be evenly tightened to specified torque.
Troubleshooting Your Holley Carb (8)


Defective accelerator pump system.

NOTE: A quick check of the pump system can be made as follows: With the engine off, remove air cleaner and look into the carburetor bores and observe pump stream, while briskly opening throttle valve. A fuel stream of fuel should emit from pump jet and strike near the center of the venturi area.


Remove air horn and check pump cup. If cracked, scored or distorted, replace the pump plunger.


Check the pump discharge ball for proper seating and location. The pump discharge ball is located in a cavity next to the pump well. To check for proper seating, remove air horn and gasket and fill cavity with fuel. No “leak down” should occur. Restake and replace check ball if leaking. make sure discharge ball, spring and retainer are properly installed.


Check pump discharge as above. Inspect diaphragm, replace if defective. Check pump inlet ball valve clearance. Adjust pump operating lever clearance.

Dirt in pump passages or pump jet.Clean and blow out with compressed air.
Fuel level.Check for sticking float needle or binding float. Free up or replace parts as necessary. Check and reset float level to specification.
Leaking air horn to float bowl gasket.Torque air horn to float bowl using proper tightening procedure.
Carburetor loose on manifold.Torque carburetor to manifold bolts (100 inch pounds).
Troubleshooting Your Holley Carb (9)


Carburetor throttle valve (s) not going wide open. (Check by pushing accelerator pedal to floor).Adjust throttle linkage to obtain wide open throttle in carburetor.
Dirty or plugged fuel filter(s).Replace with a new filter element.
Power system not operating.PISTON TYPE –

Check power piston for free up and down movement. If power piston is sticking check power piston and cavity for dirt, or scores. Check power piston spring for distortion. Clean or replace as necessary.


(Video) Holley Carb Troubleshooting and Rebuild - Things to Check!

Check power valve channel restrictions. Clean if necessary.

Float level too low.Check and reset float level to specification.
Float not dropping far enough into float bowl.Check for binding float hanger and for proper float alignment in float bowl.
Main metering jet(s) dirty or incorrect part.(1) If main metering jets are plugged or dirty and excessive dirt is in fuel bowl, carburetor should be completely disassembled and cleaned.

(2) Check the jet(s) for being the correct part. Cross reference against Holley specifications. The last two digits stamped on the jet face are the same as the last two digits of the part number.

Troubleshooting Your Holley Carb (10)


Engine needs complete tune-up.Check engine compression. Examine spark plugs (if dirty or improperly gapped clan and re-gap or replace). Check ignition point condition and dwell setting. Readjust ignition points if necessary and check and reset ignition timing. Clean or replace air cleaner element if dirty. Check for restricted exhaust system and intake manifold for leakage. Make sure all vacuum hoses are connected correctly. Make sure emission systems are operating properly.
Choke valve not fully opening.(1) Clean choke and free up linkage.

(2) Check choke thermostatic (bi-metal) coil for proper adjustment. Reset to specifications.

Fuel leaks.Check fuel tank, fuel lines and fuel pump for any fuel leakage.
Main metering jet defective, loose or incorrect part.Replace as necessary.
Power system in carburetor not functioning properly. Power valve or piston sticking in up position.Free up or replace as necessary.
High fuel level in carburetor or carburetor flooding.(1) Check for dirt in the needle and seat. If defective, replace needle and seat assembly with Holley matched set.

(2) Check for fuel loaded float.

(3) Re-set carburetor float to specifications.

(4) If excessive dirt is present in the carburetor bowl, the carburetor should be cleaned.

Fuel being pulled from accelerator pump system into venture through pump jet.Run engine at RPM where nozzle is feeding fuel and observe pump jet. If fuel is feeding from jet, check pump discharge ball for proper seating. This is done by filling cavity above ball with fuel to level of casting. No “leak down” should occur with discharge ball in place. Re-stake or replace leaking check ball, defective spring or retainer as required.
Air bleeds or fuel passages in carburetor dirty or plugged.(1) Clean carburetor or overhaul as necessary.

(2) If gum or varnish is present in idle or high speed air bleeds they can be cleaned with lacquer thinner or choke solvent in a spray can.

Troubleshooting Your Holley Carb (11)


Choke valve fully or partially open, binding or sticking.Free up with choke solvent. Realign or replace if bent.
Accelerator pump not operating properly.(1) Remove air cleaner and observe pump discharge. Replace pump cup or diaphragm.

(2) Readjust pump to specifications.

(3) Restake or replace pump intake or discharge valve.

Old or dirty (fouled) spark plugs.Clean or replace spark plugs.
Old or cracked spark plug wires.Test with a scope if possible or observe wires on dark night with engine running. Replace wires.
Partially clogged fuel filter.Replace filter on regular maintenance schedule.
Backfire on deceleration. Defective air pump diverter valve.Check hoses and fittings for tightness and leakage. Disconnect valve signal line. With engine running a vacuum must be felt. If valve or hoses are defective, then they must be replaced.
Troubleshooting Your Holley Carb (12)


Sticking throttle valves.(1) Readjust secondary throttle valve stop screw.

(2) Throttle valves nicked or throttle valve shaft binding.

(3) Repair or replace throttle valve.

(4) Check throttle body for warpage.

(5) Torque throttle body screws evenly.

Ruptured or leaking secondary diaphragm.Inspect diaphragm. Replace or install properly.
Venturi vacuum ports plugged.Try cleaning ports with choke solvent or lacquer thinner. It may be necessary to remove the diaphragm assembly and back blow into the venturi.

Troubleshooting Your Holley Carb (13)

(Video) MOST COMMON Misadjusted Holley Carburetor Setting (2 second Fix)


What causes a carburetor to stumble on acceleration? ›

If your car or truck is equipped with a Holley four barrel carburetor and stumbles upon acceleration, more than likely the carburetor needs an adjustment to the accelerator pump assembly. This assembly delivers an initial shot of fuel in the right amount, and duration, to provide crisp throttle response right off idle.

How do I know if my vacuum secondaries are opening? ›

Lower on the shaft. And if the paper clip moves we know our secondaries are working and if it doesn'

What causes a flat spot in a Holley carburetor? ›

If carburetor has vacuum piston which provides richer mixture at part throttle and moderate engine speed by opening an additional passage or jet within carburetor, a flat spot will occur if fuel valves fail to work, or fuel passages are restricted, or if piston does not function because it is sticking, vacuum leakage ...

Where are the air bleeds on a Holley carb? ›

The air bleeds are small restrictions located on top of the carburetor. Inside the air horn area on

What is the most common problem with a carburetor? ›

Problems that are often blamed on a "bad" or "dirty" carburetor include hard starting, hesitation, stalling, rough idle, flooding, idling too fast and poor fuel economy.

What causes a carbureted engine to hesitate? ›

The Most Common Causes of Hesitation:

Ignition system problem. Fuel system problem. Vacuum leak. Bad accelerator system (carburetor models)

When should vacuum secondaries open? ›

Most double-pumper carbs are equipped with a progressive linkage that starts opening the secondaries when the primaries are about 40 percent open. There are kits that allow a quicker or later opening for fine tuning a road-race or dirt track car that is often on and off the throttle.

Which is better mechanical or vacuum secondaries? ›

Holley-type vacuum-secondary carbs are more forgiving compared with mechanically actuated double-pumpers; relatively few vacuum-secondary carb sizes can be made to work on a wide variety of applications (common vacuum-secondary carbs come in 600-, 750-, and 850-cfm varieties).

How do you adjust secondary vacuum? ›

How To Adjust The Secondary Speed Screw On Holley Carbs - YouTube

How do you get rid of a flat spot on a carburetor? ›

Remove flat spot from Mikuni carburetor Chinese pit bike - YouTube

What causes off idle stumble? ›

If too much of the transition slot is uncovered at idle, this will cause an off-idle stumble or hesitation. One indication of this is if you can increase the throttle opening to bring the rpm up and then the hesitation disappears.

How do I know if my Holley accelerator pump is bad? ›

Rough acceleration

If there is any problem with the pump, then there will be a problem in the fuel mixture during acceleration. Usually, a faulty accelerator pump will result in a momentary lean condition, which can result in rough or sluggish acceleration, and even misfires.

What do Holley air bleeds do? ›

Air bleeds, sometimes referred to as "air jets" or "air bleeders" play a vital role in the operation of your carburetor. Air bleeds are responsible for determining the amount of air that will mix with each circuit in the metering block.

How do you adjust the idle mixture screws on a Holley carb? ›

How To Adjust The Idle Mixture Screws On A Holley Carburetor

What do idle air bleeds do? ›

The air bleeds draw air into the idle fuel wells and into the main fuel wells where it emulsifies (mixes) before being discharged through the idle ports and transfer slots in the base plate as well as the boost venturii up in the carburettor's main body.

What are the signs of a carburetor going bad? ›

Common signs include reduced engine performance, black smoke coming from the exhaust, backfiring, overheating, or hard starting.

What causes hesitation under load? ›

Hesitation is when your engine misfires, stumbles or lacks power when you accelerate or step on the throttle. The problem often means the air/fuel mixture is not being properly enriched or is going lean, or the ignition system is weak and is misfiring when the engine comes under load or the air/fuel mixture goes lean.

Why does my engine hesitates when I accelerate? ›

An engine that is hesitating to accelerate is most likely dealing with a fuel/air mixture that is too lean. Engine's that are running inefficiently will begin to show signs like hesitation which will only become worse over time.

When I press the gas pedal my car hesitates? ›

An engine that hesitates when accelerating is either sucking too much air, not getting enough fuel, or misfiring. Here's what you might discover—and what you can do to fix the problem: Dirty air filter. The Fix: Replace the air filter.

How do you adjust a Holley secondary vacuum? ›

How To Adjust Holley Carburetor Vacuum Secondary Springs

How do you lean out a Holley carb? ›

Holley Carb Running Rich? Watch This! | Holley Carburetor Secrets

How do you adjust the accelerator pump on a Holley carb? ›

How To Adjust The Accelerator Pump Arm on a Holley Carburetor

Is a double pumper carb better? ›

Mechanical secondary carbs (double pumpers) are the better option for race cars, hot rods, and performance vehicles with manual transmission, automatic transmissions with looser torque converters, and lower (higher numerically) rear end gears.

How much horsepower can a 600 cfm carb support? ›

A 600 CFM carb can work well with engines making 250-450 horsepower depending on the combination of parts and the RPM where it makes peak power.

Can you convert vacuum secondaries to mechanical? ›

You can't convert mechanical secondary carbs to vacuum, or vice versa. Too many differences. You can get either vacuum or mechanical secondary carbs for nearly any application.

How do carburetor secondaries work? ›

The throttle blades on the secondary barrels stay closed. As rpm increases, the engine requires more air and fuel to make power, and this is when the secondary barrels kick in. As the secondary barrels begin to open, more air travels through the carburetor, supplying the engine with the air and fuel it needs.

How do you adjust a Holley 4150 carburetor? ›

Holley carburetor tuning guide ( 4150 Carbs ) Part 1 - YouTube

How do I tell how old my Holley carb is? ›

It's like a "VIN" for your carburetor and should be hand-stamped on the front of the choke tower to the right of the vent tube on most typical Holley carbs. The list number typically is four to six digits long and may or may not have a suffix number behind it. The date code will be right below it.

What causes flat spots in acceleration? ›

Engine flat spots can be linked to all sorts of issues with your car, many of which could lead to major damage to internal components if not addressed swiftly. Potential causes can include water in the fuel lines, dirty fuel injectors, faults with the engine cam belt or clogged or burnt engine valves.

How does carburetor accelerator pump work? ›

Accelerator Pump - YouTube

How do you change the cam on a Holley accelerator pump? ›

Carburetor Accellerator Pump Cams Video - Holley Carb DVD - YouTube

How do you test an accelerator pump? ›

Easiest Way To Test The Accelerator Pump On A Weedeater Carburetor

How do you check the accelerator pump in a carburetor if working properly? ›

Accelerator-pump system

Open the throttle quickly. You should see the accelerator pump system discharge a squirt of fuel into each primary barrel. The flow should continue for a few seconds after the throttle valve reaches the wide-open throttle position. If not, the accelerator pump system is defective.

Do you need an accelerator pump? ›

With no accelerator pump or one that does not work, the engine will stumble or stall when you try to accelerate. When you give the engine more throttle in order to accelerate, the mixture will lean out causing the engine to stumble.

What causes engine hesitation on acceleration? ›

An engine that is hesitating to accelerate is most likely dealing with a fuel/air mixture that is too lean. Engine's that are running inefficiently will begin to show signs like hesitation which will only become worse over time.

How do I fix lag on acceleration? ›

The lag time can be shortened by reducing distance between pedal and throttle position, which means that your foot has less distance to travel before the throttle opens up. There are generally two ways to stop throttle delay which include using a throttle response controller and a tune.

Why does my car hesitate when I step on the gas with a carburetor? ›

If you have a carburetor, you may have a bad accelerator pump or power circuit. The Fix: Replace accelerator pump or clean or replace the carburetort. The fuel filter may be clogged. The Fix: Replace the fuel filter.

What does it mean when your car hesitates to accelerate? ›

Answer provided by. If your car hesitates when accelerating at low speed, you likely have a lean air to fuel mixture. A lean air to fuel mixture means that the gas getting to your engine doesn't have the correct mix of fuel and air.

Why is there a delay when I press the gas? ›

This delay is common in what is known as drive by wire engines. This means that the throttle is controlled by sensors and an electric motor. Most of the time when there is a delay that is this serious, it is caused by a programming issue or by the throttle body failing or being dirty.

When I press the gas pedal it won't accelerate? ›

Your Air Filter Is Dirty or Clogged

Internal combustion requires oxygen, so if your air filter is clogged, the engine may not be getting enough air to complete combustion. This can cause a decrease in performance and efficiency, which will manifest itself as your car not accelerating when you press the gas pedal.

What causes engine stumble? ›

An engine that stumbles while you are driving is either misfiring, not getting enough fuel, or is getting too much air.

What causes hesitation at low rpm? ›

Any air leakage creates engine hesitation at low RPM. Faulty fuel system: This includes your fuel lines, pump, filter, pressure regulator, and injectors. If any of these parts fail, it will cause hesitation.

Why is my rpm high but slow acceleration? ›

Fuel Filter Clogging

Another reason for your car not accelerating on high RPMs, regardless of high RPMs or not, is a dirty or even clogged fuel filter. Fuel filters are installed in a car's fuel system to filter out the dirt and the gunk in fuel.

What causes off idle stumble? ›

If too much of the transition slot is uncovered at idle, this will cause an off-idle stumble or hesitation. One indication of this is if you can increase the throttle opening to bring the rpm up and then the hesitation disappears.

What causes car to backfire and sputter? ›

An engine backfire occurs whenever the air-fuel mixture in your car combusts somewhere outside the engine's cylinders. This can cause damage to your car's exhaust or intake if left unchecked -- and it also means that your car's engine isn't making as much power as it should, and is wasting lots of fuel.

What causes lean air/fuel mixture? ›

A lean mixture occurs when there's a higher concentration of air to fuel than there should be.


1. Holley Carb Choke Problems And Removal | Holley Carburetor Secrets |
2. Holley Carb Flooding: The NOT immediately Obvious Problem!
3. Common Holley Carb Leak Solutions | Holley Carburetor Tuning Secrets |
4. Troubleshooting - backfire through carburetor
5. Holley carburetor stumble fix
6. 5 Secrets For Tuning And Adjust Holley Carbs That The Pro Builders Wont Tell You | Carb LS |NW Ep.60

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