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.
Small hardware can make a big difference. Displayed here is an assortment of Holley upgrade accessories, including six pump cam options, two upgrade shooter screws and four modified shooters.
One area that seldom requires adjustment (but is sometimes inadvertently "fixed") is the accelerator pump lever. Its adjustment nut is pre-set by Holley and shouldn't be touched. When properly set, the arm will not bottom out on the pump housing and will feature at least 0.015 inch of extra travel.
The accelerator pump shooter is visible at the top of the carburetor, beneath the indicated Phillips-head screw. It's changed by removing the Phillips-head screw. If changing shooter sizes, be careful with the tiny gaskets?there is one gasket on top of the shooter and one below it.
Numerous shooter shapes and sizes are available for high-performance Holley applications. The difference between shooters with tube extensions and those without is not terribly consequential, but you shouldn't let this stop you from testing both. They each have their fans.
Drilling main jets is unacceptable, but it'll work for shooters. The best way to drill them is with a common pin-vise. If you're using modified shooters, be absolutely certain that you double-check the bore size before swapping the shooter (the size markings will be useless if you redrill the orifice). Figure out a method of keeping track of shooter modifications.
Accelerator pump cams can be swapped with those of different profiles (change in duration and lift; you know the drill). In addition, the cams have two different mount locations. This allows you to toy with the pump cam without swapping it for another. To change location of the cam, the small screw is removed and then replaced in the second hole.
The holes in each accelerator pump cam are visible here. While not shown, yellow and brown pump cams are dedicated 50cc REO models while others can be used with conventional accelerator pumps. To match the higher-volume 50cc pump, REO cams have more lift and duration than the standard versions.
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