Carb Science Series — Accelerator Pump Tuning for Holley Carburetors

It’s an early July evening, and you’re in the shop with a buddy working on your car. All the doors and windows are open when you hear the sound of some distant small-block losing the battle to accelerate. The engine stumbles, coughs, and then accelerates. You both look at each other and nod knowingly…

We’ve all been there. The classic off-idle stumble causes the engine to hesitate, or worse, sneeze into the air cleaner. It’s embarrassing, especially if you’re friends are nearby, like the party foul nobody wants to make. You are on your own with social blunders, but we can help with curing those accelerator pump problems.

This story will focus on how to avoid these Holley carb-sneeze issues. What makes this even easier is that Holley offers a phalanx of parts to help with the tuning process. We’ll run through a basic setup so you can at least start with something close to what your carb needs, and then get into some more extensive tuning exercises. Limber up those wrench-turning fingers, and let’s get started.

The recommendations in this story apply to both vacuum-secondary carburetors (left) and mechanical-secondary carbs that use accelerator pump circuits on both the primary and secondary sides.

Carburetor Detective

Let’s assume we have an average 350ci small-block Chevy making around 450 horsepower with an older Holley 750 vacuum-secondary carburetor, mild cam, dual-plane intake, and headers. The engine idles at 13 inches of vacuum, there are no vacuum leaks, and the engine is equipped with an excellent ignition system with 14 degrees of initial timing and a nice advance curve.

With all that going for it, the engine stumbles just off-idle. Our first tuning efforts should be to ensure that both primary and secondary float levels are correctly set, and the accelerator pump nozzle is close to the stock size. In this case, that would be a “31” (0.031-inch) nozzle.

This underside view reveals the entire accelerator pump system, including the throttle linkage (1), accelerator pump cam (2), pump linkage (3), and accelerator pump cover (4).

Making the proper diagnosis is a little bit like detective work, looking for clues. Older Holleys that have been around for a long time tend to suffer abuse from “tuners” who like to fiddle with a carburetor with no realization of the consequences. A prevalent issue with Holleys is excessive clearance between the accelerator pump linkage and the arm extending from the accelerator pump cover.

A quick test to determine this is to remove the air cleaner and move the throttle linkage slowly while watching the accelerator pump arm. The instant the throttle linkage moves, the accelerator pump arm to the cover should also move.

It’s common to find the throttle linkage moves 10 to 15 degrees before the accelerator pump linkage begins to move. That’s your big clue. Even better, the fix is both quick and easy. It indicates that, for whatever reason, someone has tightened the adjustment on the linkage. The proper adjustment is to set the spring-loaded linkage so there is minimal clearance between the arm and the throttle linkage at curb idle.

This is where many simple drivability and part-throttle hesitation issues can be solved. Adjust the linkage so that when the throttle linkage moves, it also imparts movement of the accelerator pump circuit. It should only require a minor change in length and should be checked anytime a fuel bowl is removed. The secondary side accelerator pump should be similarly adjusted.

One reason for needing adjustment is that the pump arm alignment can change when the float bowl is removed for tuning or the accelerator pump cam is changed. Every time the float bowl is removed and replaced, this accelerator pump adjustment should be verified.

When the linkage is accurately set, the circuit will begin to squirt fuel from the accelerator pump very soon after the throttle is opened even a small amount. A minor clearance at curb idle between the arm and the adjuster is acceptable.

One reason for the tall spring and adjuster bolt is to prevent linkage damage, should something obstruct or prevent this hydraulic system from pumping fuel. If this happens, the long spring will compress, still allowing the throttle to move but not bend the linkage.

A couple of different sizes of accelerator nozzles, plus a pump cam kit, would probably cover most tuning requirements. There are nine different cams in Holley’s accelerator pump cam assortment kit, and these can be changed quickly for testing.

Next on the Troubleshooting Checklist

Let’s say we’ve properly adjusted the linkage and light acceleration has improved, but when the throttle is mashed, a hesitation still exists. For this situation, let’s say the accelerator pump nozzle – often called a squirter – is a much larger “48.” Remember, we’re looking at a carburetor that has been around the block a few times — perhaps several zealous tuners have performed their “magic” — and we now find a larger nozzle compared to the stock 0.031-inch size.

We’ll change this back to the stock size and see what happens. In this case, the plan works, and we discover that we can install a nozzle smaller than stock, down to a “28.” The lesson here is that too-large of a nozzle can create a hesitation that will feel much the same as one that is too small.

Holley offers a broad range of sizes of accelerator pump nozzles. This is a 31 – note the hollow screw that can be used anywhere but is important with nozzles larger than 0.040-inch. Most street carburetors will operate with nozzle sizes ranging from 0.028 to 0.037-inch. Be sure to include the small, round gaskets on both sides of the nozzle.

Another vital point is that the tuner should always shoot for the smallest nozzle the engine will accept. Nozzle size controls more than just the initial volume of fuel — it also determines the duration of the shot. At wide-open-throttle (WOT), a smaller nozzle size lengthens the duration of the pump shot compared to a larger nozzle. This may become important in different applications.

For example, with a manual transmission, full clutch engagement in everyday street driving loads the engine heavily. It may require a slightly larger nozzle size to compensate, especially if the engine is equipped with a large, single-plane manifold. Conversely, an automatic with a loose converter will not load the engine nearly as aggressively, perhaps permitting a slightly smaller pump shot.

Nearly all Holley accelerator pump circuits use this needle directly underneath the discharge nozzle. It acts as an anti-drain-back valve to retain fuel in the circuit above the float level. Sometimes these valves can stick. If so, remove the nozzle and gently pry on the top with a tiny screwdriver. Then replace the nozzle and operate the linkage. Don’t hit the throttle with the nozzle removed. If you do this, fuel will shoot the needle up and likely drop past the throttle blades and right into the intake manifold. Ask us how we know that…

Cam Swap

Another option available for the tuner is the accelerator pump cam. This is the plastic eccentric located on the throttle shaft. Holley offers a replacement kit with eight different color-coded pump cams. The most commonly used accelerator pump cam profile is the white version.

Holley includes a graph in the catalog we’ve reproduced for this article that shows how each of the cams alter the lift curve. A majority of the cams only begin to change dramatically after 20-degrees of throttle opening. However, several cams are more aggressive, with the purple and yellow cams offering the most lift.

Another quick-and-easy option to alter the lift curve is the position options on the throttle shaft. A close look at the throttle linkage arm reveals two positions available to mount the pump cam. Moving from the stock position (number 1) to the number-2 position advances the cam slightly, which makes that same cam more aggressive by altering its starting point. This is a small change but can be used for minor tuning advancements.

This is a reproduction of the Holley accelerator pump cam lift curve. Throttle position is listed across the bottom (X-axis), while the pump cam lift is listed on the vertical (Y-axis). The white curve is the most commonly used cam. The yellow pump cam is a separate part number (20-81) intended for Dominator carburetors.

While tuning the accelerator circuit can certainly help dial-in any Holley, it’s also important to mention that all of these changes do little to help if the engine is not in proper tune. A sharp carb tuner whose name is lost to history once said: “Ninety-percent of carburetor problems are ignition related.”

That number might be slightly inflated. Still, in many cases, a problem that appears to be carburetor related is eventually traced to ignition difficulties such as insufficient or non-existent mechanical advance, too little timing, too much timing, fouled spark plugs, bad spark plug wires, and a host of other ignition maladies.

A popular technique when addressing a carburetor related issue is to first return the carburetor to stock jetting, float level, and accelerator pump specs. Holley lists the stock specs for all of these systems and much more in the catalog.

Through the years, we’ve found dozens of carburetors that didn’t run correctly because they had been “tuned” to the point of being undrivable. Many times just returning the carb to stock specs improved the situation. At least with stock specs, we can assume the carb is close to functioning properly and then adjust from there.

A quick way to enhance the lift curve on any cam is to move the cam mount from the Number-1 to the Number-2 position. This starts the lift curve sooner.

Article Sources

About the author

Jeff Smith

Jeff Smith, a 35-year veteran of automotive journalism, comes to Power Automedia after serving as the senior technical editor at Car Craft magazine. An Iowa native, Smith served a variety of roles at Car Craft before moving to the senior editor role at Hot Rod and Chevy High Performance, and ultimately returning to Car Craft. An accomplished engine builder and technical expert, he will focus on the tech-heavy content that is the foundation of EngineLabs.
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