Troubleshooting Your Holley Carb

Troubleshooting Your Holley Carb

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

(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
Engine cranks over, but will not start when cold: Check the fuel filter.


Improper starting procedure used Check to determine if proper starting procedure is used, as outlined in owner’s manual
No fuel in gas tank Add fuel. Check fuel gauge for proper operation.
Choke valve not closing sufficiently when cold Adjust the index of the choke thermostatic (bi-metal) coil
Choke valve or linkage binding or sticking Realign 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.


Troubleshooting Your Holley Carb
Engine starts hard when hot: Check the fuel pump.


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
Engine starts and stalls: Check the float level.


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
Engine idles rough and stalls: Correct idle speed setting.


Idle mixture adjustment Adjust 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
Engine runs uneven or surges: Check for vacuum leaks.


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
Engine hesitates on acceleration: Check the accelerator pump system.


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
No power on heavy acceleration or at high speed: Check the jets.


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.


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
Poor fuel economy: Check the power valves.


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
Engine backfires: Clean or replace spark plugs.


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
Secondaries don’t open: Sticking throttle plates.


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

4 Comments on Troubleshooting Your Holley Carb

  1. I need some help…I was running the car at high idle and then the motor just stopped. I can start the car for a few seconds by pumping the the throttle but it quickly dies. Using a 600 CFM carb, 80457. I found the primary side had a bad seal on the bowl at the top. I replaced a put it back together and tested with the same issue. I can race of engine for a few seconds and quickly dies. Please help???!

  2. I am running a new mild build 350 with a new holly street avenger 670. When I first start the motor, it runs great at first but, when it warms up and I finally come to a stop 10 miles the motor starts to sputter when I hit the accelerator. once I slowly get going and come to another stop it may run great or it sputters again. I am stumped as to what the problem may be

  3. My car was running fine going to the store. Came out about 15 minutes later to a no start. towed the car home and discovered that I’m not getting any fuel into my front bowl. (Holly 750 4779 double pumper)I checked the float to see if it was sticking, it’s fine. I have 7 lbs fuel pressure going in both bowls. Not knowing a lot about carbs, I don’t want to go any further without some guidance. Thank you

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