How To Cure Carburetor Stumble
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. Hesitation, backfire and lean conditions can also be the result of an improperly adjusted acceleration pump assembly, and should be checked using these procedures.
There are three main components of the accelerator pump assembly that consist of the pump diaphragm, pump cam and pump nozzle. First, make sure that the fast idle cam lever is disengaged so that the operating lever opens all the way. Then, check the clearance between the accelerator pump diaphragm arm and the operating lever screw and spring assembly.
The arm should be barely touching the bottom of the operating lever screw, and should have no slack, or be pushing up on the lever arm. Insert a 0.015 feeler gauge between the pump arm and the adjustment screw bottom. The pump arm should move down a bit when the feeler gauge blade is inserted. The proper adjustment prevents the diaphragm from getting stretched to its maximum limit at wide open throttle. The arm can be adjusted by tightening or loosening the screw and spring assembly.
Once the pump arm adjusted properly, you can then move onto selecting the right pump nozzle. Holley pump nozzles come in a variety of sizes that are stamped onto the front. Typically, heavier vehicles with low numbered axle ratios (like 3.23:1) will use a smaller nozzle. Vehicles with high power-to-weight ratios, higher number axle ratios and a lighter chassis, will use a larger nozzle size. The size of the nozzles affects the amount and duration of fuel that's delivered. Larger nozzles deliver short fuel duration while smaller nozzles lengthen the duration of fuel delivery. The overall objective is to deliver the right amount of fuel. You don't want to deliver too much fuel too soon, or to little fuel too late. That's what causes the bogging and poor throttle response.
Start by increasing the fuel nozzle size up by three sizes. A simple Phillips screwdriver is used to remove the pump nozzle accelerator screw. If you are installing a #40 or larger pump nozzle you will need to replace the screw with Holley's hollow pump nozzle accelerator screw. This allows maximum fuel to flow through the nozzle. Be careful not to loose the check valve needle that sits under the nozzle. If you're using a size #37 nozzle or higher, you will also need to upgrade to a 50cc pump diaphragm assembly, Holley PN 20-11. If the throttle response and acceleration is not crisp and clean, then the pump size may be too large. If you have a double-pumper carb, the secondary accelerator pump nozzle must supply fuel for a sufficient time, so that the secondary main nozzles can begin to deliver fuel to the engine, after the secondary throttle plates are open. Otherwise this will also cause the engine to bog.
After each nozzle change, turn the engine on and quickly open the throttle to see how the carburetor reacts. Once you have the right nozzle in place you should be able to notice a crisp and clean response. Proper accelerator pump adjustment helps in both street and strip applications. For more information, vist www.holley.com. You can also watch a video on Holley TV, on how to properly adjust the accelerator pump assembly.
Text by: DS Media Relations
A Look Inside Your Holley Carburetor: Tuning Tips- PDF
Carburetor Installation And Tuning DVD
Adjusting The Accelerator Pump Assembly
Start by checking the adjustment of the accelerator pump diaphragm arm adjustment screw. Use a 0.015 feeler gauge to make sure the adjustment is correct. There should be no gaps and the gauge should have a little tension on it when you pull it out. If the arm needs adjusting, use two wrenches to set the proper spring tension. Accelerator pump nozzles come in a variety of sizes. Start by increasing the fuel nozzle size up by three sizes.
Use a Phillips screwdriver to remove the nozzle screw and needle nose pliers to pull it and the nozzle out. Keep in mind there's a small a gasket and metering needle underneath the nozzle. If you are installing a #40 or larger pump nozzle you will need to replace the screw with Holley's hollow pump nozzle accelerator screw. After each change, start the engine and check the throttle response. The goal is to deliver the right amount of fuel at the right time. You don't want to deliver too much fuel too soon, or to little fuel too late. That's what causes the bogging and poor throttle response.
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