The airflow through the carburetor responds immediately to any increase in throttle opening, but there is a brief interval before the fuel in the narrow passages can gain speed and maintain the desired balance of fuel and air. The accelerating pump system operates during this interval, supplying fuel until the other systems can provide the proper mixture. When the throttle is closed, fuel enters the pump well through a passage from the float chamber. The pump rod, which is link to the throttle lever, moves downard as the throttle is opened. The horizontal arm at the top of the pump rod slides down in the slot in the pump iston stem, compressing the pump spring. The spring presses the pump piston down, forcing fuel through the pasage to the pump discharge needle valve. This needle valve prevents the siphoning or “pullover” of fuel from the pump well. The fuel, under pressure from the pump piston, unseats the needle valve and flows past it into the pump discharge nozzle. The narrow diagonal passage at the end of the nozzle is a calibrated restriction which meters the fuel as it is sprayed into the airstream. The ball check valve in the pump inlet prevents a reverse flow of fuel from the pump well to the float chamber when the pump is operated. The spring overriding feature provides a sufficiently long interval of discharge, regardless of how suddenly the throttle is opened.
If you’re having carburetor trouble and suspect the accelerator pump, do a quick accelerator pump test first.
Then, check out this video for more detailed testing
The throttle shaft lever will generally have 3 holes, sometimes 4.The center hole is for the standard setting. As you go towards the float bowl you get richer and leaner as you go the other way. Adjust the setting to the best condition after driving the vehicle.
Holley 94, 2100, 2110 Check Weight
Holley Accelerator Pump Cup AP500
