Carburetor Tuning Guide - Twisting Knobs On Holleys

The tools you need: A vacuum gauge, a couple of flat-blade screwdrivers, a Phillips screwdriver, and a 5/8-inch combination wrench. Not shown is a 0.015-inch feeler gauge.

Float setup is first. There are a number of theories about the right and wrong way to adjust floats on a Holley carburetor. One of the accepted methods works like this: On a running engine, set the primary floats so that fuel just seeps out of the sight plug. This means that the side of the bowl should be "wet" as you set the float level.

On the secondary side the sight plug is situated lower on the bowl. In this case, set the floats slightly higher. Occasionally, you'll have to rock the car to get fuel to seep out. By the way, an absorbent shop towel placed on the intake manifold below the sight plug is a good idea to catch wayward gasoline.

To set the floats, loosen the slotted lock screw at the top of the fuel bowl, and then turn the large adjustment nut (which actually moves the needle-and-seat assembly up and down). Retighten the lock screw once the adjustment has been finalized. Keep an eye on the sight plughole. Remember, the fuel shouldn't pour out. Several companies offer clear sight plugs that allow you to see the float level without actually removing the plug. That can be a bonus, since raw fuel won't pour onto a hot intake manifold.

With the float level set, you can fine-tune the accelerator pump discharge nozzle (shooter). All Holley-equipped Corvettes have at least one shooter. Aftermarket mechanical-secondary Holleys (as well as the carbs on L88 and ZL1 Corvettes) were double pumpers, with two accelerator pumps and two shooters. If the Corvette's initial acceleration produces a hesitation and then picks up, the shooter size must be increased. In some cases, the accelerator pump shooter may be so small it creates a lean condition, and the engine will backfire during acceleration. If the shooter is too large, the off-idle acceleration will not be crisp or clean, and there may be a puff of black smoke out of the headers during acceleration. Holley offers many sizes, shapes, and configurations of shooters. The two high performance models include the tube discharge jobs and the straight-end discharge versions. All shooters are numbered from 25 through 52; the numbers indicate the drill size of the shooter orifice in thousandths of an inch

Accelerator pump shooters are attached to the main body of the carburetor in the venturi area (in this location, behind the bowl vent) with a Phillips-head screw.

They're easy to see in this photo of a Holley race carburetor (which has no choke). To swap the shooter, loosen and remove the screw that holds the shooter in place.

Next, install the new shooter(s). Ensure the throttle plates are closed. Before you drop them into place, be sure that the needle check-valve is in the shooter discharge passage. The check valve installs pointy-side down. The shooter can now be installed; be sure to install the hardware in order. From left to right: shooter screw, check valve, shooter screw gasket, shooter (or discharge nozzle), discharge nozzle gasket.

A small piece of hardware that's often overlooked is the pump shooter screw. Any time the shooter size goes beyond 0.040-inch, it's wise to add a hollow shooter screw kit. By using a hollow screw, you've insured that the limiting restriction in the accelerator pump system is, in fact, the shooter, not the screw. Holley sells hollow screw kits; the part number for the screw is 121-7.

A companion to the shooters is a series of replaceable accelerator-pump cams. Holley pump cams have varied lift and duration profiles, just like an engine camshaft. The specific cams are color-coded, and each cam has two possible mounting positions. These positions change the duration of the pump shot. Holley recommends the pump discharge nozzle (shooter) be tuned first, then spend the time to fine-tune the cams. There is no "correct" cam for a given engine. Like the shooters, the pump cams require experimentation to determine which cam is best suited for your particular combination. Holley sells a pump cam assortment under part number 20-12.

Pump cams are located on the throttle shaft. Remove the screws that hold the pump cam (or cams in the case of a double-pumper) in place. Note the location of the cam (position 1 or position 2), what the color is, and the type (standard 30cc or 50cc REO type). Installation is just the opposite. Keep in mind that pump cams designed for use with 50cc pumps are different from those designed for 30cc pumps. They can't be interchanged. Also, the big 50cc REO pumps never came standard on any production line Corvette carburetors.

The accelerator pump override spring can also be adjusted, though if the carburetor hasn't been messed with, these springs are regularly within the specification range.

Holley specs call for a clearance of 0.015-inch. Open the choke and unscrew the idle speed adjuster so that the primary butterflies are fully closed (the fast idle cam cannot hold the throttle open). Turn the carburetor upside down and fully open the throttle. Place a 0.015-inch feeler gauge between the pump cam arm adjusting screw and the pump lever. If there isn't sufficient clearance, tighten the override screw (nut) until the pump lever moves in conjunction with movement on the cam follower arm. If there is too much clearance, loosen the adjusting nut. Remove the feeler gauge, close the throttle, and check the pump cam arm for any play. If any free play exists, the pump cam arm could be bent. The idea is to have 0.015-inch clearance at full throttle and no free play in the pump cam arm when the throttle is closed.

Next is setting up the idle. If your Corvette has a big duration camshaft (and a very lumpy idle), you'll find the idle mixture screws tend to be unresponsive. There's an easy fix: Adjust the secondary throttle stop screw. Holley four barrels have a small setscrew located beside the passenger-side throttle-bore shaft bore (on the base plate of the carburetor). By turning the screw clockwise approximately 11/2 turns, the throttle blade on the secondary will open slightly. This results in 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 butterflies adjusted with a small length (approximately 0.020-inch, appearing as a square) of the idle transfer slot visible below the bottom edge of the butterflies. If the secondary transfer slots (as shown) are uncovered excessively, the engine can develop an undesirable hesitation when the butterflies are suddenly opened.

When you have adjustment sensitivity on the idle mixture screws, you can then balance the idle mixture. To accomplish this, install a vacuum gauge on the engine. Pick one idle mixture screw and call it "number 1." Set this screw until you reach the highest manifold reading on the gauge.

Then proceed to the second mixture screw and set it to the highest reading. Go back and repeat the process, fine tuning the idle mixture, and repeat the process for a third and final time. If your aftermarket Holley is equipped with four-corner idle circuits, set the front first, then set the rear. Recheck the front idle after the rear setting has been confirmed. Reduce the idle speed and repeat the process, perhaps several times, until the engine idles at the desired engine speed. If the carburetor has never been used previously, turn the idle mixture screws all the way in, and then back them out in unison. For the initial adjustment, all 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.

Once the correct idle mixture has been established, reset the idle speed. In most cases you'll find that balancing the idle mixture will increase the idle speed considerably. Reduce the speed to an rpm range that allows the engine to idle properly. If the Corvette is equipped with an automatic, it should be capable of idling slowly in gear. Keep in mind that "slow" could mean an idle speed of 1,000 rpm if a large duration camshaft with a lot of overlap is used.