Holley double pumper carburetors and idle tuning tools

Holley four-barrel carburetors are probably the most common high performance fuel mixing devices on the planet. Given the sheer numbers of carburetors made since the 1950s, you know they work. If there is a fly in the Holley ointment, it’s setting up the idle. Since Holleys are so easy to adjust, it’s also easy to adjust them totally wrong. People will get the idle settings wrong, then blame the carburetor when it doesn’t work properly. That’s a big mistake.

In reality, setting the basic idle on a Holley isn’t difficult. We’ll show you how to do it on both vacuum secondary and double pumper models. What works on this pair of carburetors will work on your high performance Holley, too.

We’ll take you through the entire process in the slide show below:


Tools for Adjusting Carb Idle
Adjusting Idle Mixture Screws
Carb Idle Adjustment
Adjusting Idle Mixture
Four-Corner Idle Circuits
Adjusting Idle Mixture

Here are the tools you’ll need (and it’s not much): a vacuum gauge and a medium size, flat blade screwdriver.

If your car or truck has a camshaft with considerable duration (and as a result, a very lumpy idle), adjusting the idle mixture screws may not help very much. There’s an easy fix: adjust the secondary throttle stop screw. Holley four barrels like this 750 cfm 0-3310C have a small set screw located beside the passenger side throttle bore shaft bore (on the base plate of the carburetor, so you’ll have to flip it over). By turning the screw clockwise approximately 1-1/2 turns, the throttle blade on the secondary will open slightly. This allows a rather large amount of airflow through the carburetor without increasing the “window” seen by the idle transfer slot. At this time, the primary throttle blades can be closed, which tends to enhance the sensitivity of the idle mixture control.

Most carburetors will have the secondary butterflies adjusted with a small length (approximately 0.020-inch, appearing as a square) of the idle transfer slot visible below the bottom edge. If the secondary transfer slots are uncovered excessively as shown here, the engine can develop an undesirable hesitation when the butterflies are suddenly opened. Keep that in mind when adjusting the secondary stop.

With the secondary throttle stop set, the idle mixture screws should be more responsive. Now you can balance the idle mixture. Connect a vacuum gauge to a manifold vacuum source on the engine. Starting with one idle mixture screw, turn it until you get the highest manifold vacuum reading on the gauge. Do the same thing with the other mixture screw. Repeat the process twice more to fine-tune the idle mixture.

If your aftermarket Holley is equipped with four-corner idle circuits like this 850 cfm 0-4781 double pumper, set the front (primary) circuit first, then the rear using the procedure outlined in the previous caption. For the initial adjustment, all idle screws should be within 1/4-turn of each other, although you may have to turn the primary screws out an additional 1/8 to 1/4-turn. Recheck the front idle mixture after the rear setting has been set. Reduce the engine idle speed and repeat the process until the engine idles at the desired rpm. If the carburetor is new or freshly rebuilt, turn the idle mixture screws all the way in, then back them out in unison.

In most cases you’ll find that balancing the idle mixture will increase the idle speed considerably. Reduce the speed to an rpm setting that allows the engine to idle properly. If the vehicle is equipped with an automatic, it should be capable of idling slowly in gear. If you have a large duration camshaft with a lot of overlap, “slow” could mean an idle speed of 1,000 rpm.



Author: Wayne Scraba

Wayne Scraba is a diehard car guy and regular contributor to OnAllCylinders. He’s owned his own speed shop, built race cars, street rods, and custom motorcycles, and restored muscle cars. He’s authored five how-to books and written over 4,500 tech articles that have appeared in sixty different high performance automotive, motorcycle and aviation magazines worldwide.