Properly setting the idle screws on a Holley carburetor is one of the most commonly overlooked maintenance task. The idle screws determine the ratio of air and gasoline mixed by the carburetor when the engine idles. If these screws are not properly adjusted, the engine will often idle poorly and the valves and pistons can become coated with carbon deposits. Adjusting these idle screws is fairly simple and takes only a few minutes.
Locate the idle mixtures screws on the two metering blocks. A single metering block is located between the main body of the carburetor and the float bowl on either side of the carburetor. On the side of each metering block, there is a small screw. Together, these two idle mixture screws control the air/fuel mixture produced by the carburetor.
Turn the two idle mixture screws in a clockwise direction with a flathead screwdriver until each screw stops.
Turn each idle mixture screw one complete revolution in a counterclockwise direction with the screwdriver, then turn each screw an additional half revolution in the same direction. This is the carburetor's baseline setting. It is from this setting that the idle screws can be adjusted from.
Turn the engine on, then turn both idle screws, one at a time, in a clockwise direction one-half revolution with a screwdriver while the engine is idling. Slowly turn one idle screw in a clockwise direction until the engine's idle speed can be heard increasing, then turn the screw in a counterclockwise direction one-half revolution to set that screw. Repeat the process with the second idle screw to complete the process.
