Generally an engine will pop, miss or surge if it’s lean, although an excessively rich condition can cause the same problems. As a rule, cool, dense air requires larger jets, while hot thin air requires smaller jets. Whenever a carburetor spacer is added or removed, a camshaft, cylinder head or intake manifold change is made.

Engine performance will be poor if the accelerator pump nozzle diameter is incorrect. If a car is sluggish during initial acceleration and 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.

For oval track use, the pump lever should be adjusted so there is no play in the pump linkage when he 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 often involves reducing rather than increasing the delivery valve pump volume and discharge rate.

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 5000 RPM, the pump shot should begin at 4700-4800. An 1800 RPM launch calls for the accelerator pump shot starting at 1500. The key is to have no slop in the accelerator pump system at starting line RPM so that the pump shot is not used up below that RPM. Although adjusting the accelerator pump 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. Drag cars equipped with a stick plunger shift or transbrake, where starting line launch is accomplished with the carburetor wide open requires adjusting like an oval track application.