Holley Carburetor Tuning

Tweaking Holley accelerator pumps

One of the most botched, most misunderstood and most befuddling parts of the Holley carburetor is the accelerator pump circuit. Performance is a game of winners and losers; and, if your car dies, bogs or stumbles as you wind through the gears, you're gonna lose. A poorly tuned pump circuit can do that and can also be dangerous—especially when you have to pull out into traffic. Why, then, do carburetor experts tell us that fine-tuning a Holley accelerator pump circuit is easy?

Almost all Holley carburetors are fitted from the factory with a standard volume accelerator pump. These pumps are originally adjusted by opening the carburetor to WOT (wide-open throttle) with the engine turned off. The pump lever should not bottom out in the accelerator pump housing, but should feature at least 0.015-inches of extra travel. Don't be tempted to tighten the accelerator pump screw at the pump lever spring in anticipation of increasing pump travel and volume. This spring is pre-set and, in almost all applications, it should not be touched.

What types of shooters are available? Holley offers many sizes, shapes and configurations. The two high-performance types include the tube-discharge examples and the straight-type end-discharge jobs. Although quite different in overall appearance, there seems to be little difference in the performance of either hi-po shooter unit. All shooters are numbered from 25 through 52 (the stamped-in numbers indicate the drill size of the shooter orifice). While jets cannot (and should never) be redrilled, shooters are another matter. Redrilling the orifice size with a pin-vise drill is common practice, however the numbering "system" is then thrown out of whack. Drill the shooter if you prefer to, but always remember to physically check the orifice size before you slip it into the carburetor.

If tuning (such as revising the pump cam timing or increasing the shooter size) doesn't work, you may have to increase the size of the rear accelerator pump. Holley sells a 50cc accelerator pump kit (often referred to as the "REO kit") for this application. This part (Holley P/N 20-11) includes the body, special diaphragm, arms, cams and all mounting hardware. It should be pointed out that when shooter size is increased beyond 0.40 inch, it's wise to also use Holley's hollow shooter screw kit (P/N 26-12). This setup allows increased fuel flow to the pump shooter, ensuring that the limiting restriction in the accelerator pump system is in fact the shooter, not the screw.