#YELLOW BULLET FORUMS HOLLEY TUNING UPGRADE#

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. Be careful not to loose the check valve needle that sits under the nozzle. This allows maximum fuel to flow through 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. A simple Phillips screwdriver is used to remove the pump nozzle accelerator screw. Start by increasing the fuel nozzle size up by three sizes. That's what causes the bogging and poor throttle response. You don't want to deliver too much fuel too soon, or to little fuel too late. The overall objective is to deliver the right amount of fuel. Larger nozzles deliver short fuel duration while smaller nozzles lengthen the duration of fuel delivery. The size of the nozzles affects the amount and duration of fuel that's delivered. Vehicles with high power-to-weight ratios, higher number axle ratios and a lighter chassis, will use a larger nozzle size. Typically, heavier vehicles with low numbered axle ratios (like 3.23:1) will use a smaller nozzle. Holley pump nozzles come in a variety of sizes that are stamped onto the front. Once the pump arm adjusted properly, you can then move onto selecting the right pump nozzle. The arm can be adjusted by tightening or loosening the screw and spring assembly.

The proper adjustment prevents the diaphragm from getting stretched to its maximum limit at wide open throttle. The pump arm should move down a bit when the feeler gauge blade is inserted. Insert a 0.015 feeler gauge between the pump arm and the adjustment screw bottom. 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. Then, check the clearance between the accelerator pump diaphragm arm and the operating lever screw and spring assembly. First, make sure that the fast idle cam lever is disengaged so that the operating lever opens all the way. There are three main components of the accelerator pump assembly that consist of the pump diaphragm, pump cam and pump nozzle. Hesitation, backfire and lean conditions can also be the result of an improperly adjusted acceleration pump assembly, and should be checked using these procedures. This assembly delivers an initial shot of fuel in the right amount, and duration, to provide crisp throttle response right off idle. 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. Cure The Bog And Sluggish Response On Your Holley Four-Barrel Carburetor, By Properly Adjusting The Accelerator Pump Assembly

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