Generally an engine will pop, miss or surge if it’s lean, even though an excessively rich condition can cause the same problems. Cool, dense air requires larger jets while hot thin air requires smaller jets. Whenever a carburetor plunger spacer is added or removed, a camshaft, cylinder head or intake manifold change is made. It will be necessary to retune the carburetor to regain maximum performance.

Another problem that will cause a stumble or hesitation is when the secondary jets become uncovered, and no amount of accelerator pump tuning will heal it. This occurs when a car leaves so hard causes the secondary bowl to be pushed up against the rear wall, uncovering the jets. Whenever jet extensions are installed in a carburetor equipped with a secondary power valve, the power valve must be removed and the hole plugged. Jet size must then be increased.

Even when the jetting is within the ball park, this is no guarantee of optimal performance. 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. Fuel spilling out of the vent tubes is another possible cause. The latter problem is easily healed by running a rubber hose from one vent tube to another, with a slot cut at the top of the hose. As with jetting, determining the best nozzle diameter is accomplished by trial-and-error testing. Simply adjust size up or down for best performance.

Do not forget about the accelerator pump lever adjustment. 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 delivery valve accelerator pump for maximum performance off the corner often involves reducing, rather than increasing, the pump volume and discharge rate.