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Tech Answer

When should I consider changing an accelerator pump?

Even with correct jetting, carburetor performance will be poor if the accelerator pump nozzle (squirter) diameter is incorrect. If a car is sluggish during initial acceleration, a puff of black smoke blows out of the headers when a drag car leaves the starting line, or when an oval track car comes off a corner, the accelerator pump nozzle diameter may be too large. Fuel spilling out of the vent tubes is another possible cause. The latter problem is easily cured by running a rubber hose from one vent tube to another, so long as a slot is cut at the top of the hose. As with jetting, determining the best squirter diameter is accomplished by trial-and-error testing. Simply adjust the size up or down until the best performance is achieved. And don’t forget about pump lever adjustment. For oval track use, the lever should be adjusted so that there is no play in the pump linkage when the throttle is closed. This will assure that there will be no lean stumble when the carburetor comes off idle. Tuning an accelerator pump for maximum performance off the corner frequently involves reducing pump volume and discharge rate. Drag racing calls for a slightly different approach. For the hardest starting line launch with a foot brake, the pump lever override spring should be adjusted so that fuel starts to discharge through the nozzle at an engine speed lower than launch rpm. If a car leaves the starting line at 5,000 rpm, the pump shot should begin at 4,700-4,800 rpm. The key is to have no slop in the accelerator pump system at starting line rpm, so that the pump shot isn’t used below that rpm. Although adjusting the accelerator pump as described will create a lot of slop in the pump linkage at idle (and may produce a stumble when driving in the pits) a car will leave harder. Cars equipped with a stick shift or trans-brake, where starting line launch is accomplished with the carburetor wide open, requires adjustment like an oval track application. application.

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