How to Adjust the Air Mixture for a Holley Carburetorby John Stevens
Properly adjusting a Holley carburetor's air/fuel mixture is one of the most commonly overlooked and important considerations. A mixture which is too rich, meaning the mixture does not have the proper quantity of air, can result in fouled sparkplugs and poor gas mileage. If the mixture is too lean, meaning that the mixture has too much air, the mixture can actually destroy the engine if left uncorrected. Thankfully, adjusting the mixture on a Holley carburetor is quite simple, as the mixture screws are located on the outside of the carburetor and can be quickly adjusted with a screwdriver.
Locate the two air/fuel mixture screws on the metering blocks. Holley carburetors consist of a "body" and several components which attach to the body. The "body" of the carburetor is the large, square-shaped center section of the carburetor. At both the front and back of the body is a single float bowl. Between the body and each float bowl is a rectangular block, approximately 3/4 of an inch in width, called a metering block. On the side of each metering block is a single screw. These two screws work in conjunction to control the air/fuel mixture of the carburetor.
Set the two air/fuel mixture screws to a baseline point so that the mixture can be adjusted while the engine is idling. To set the baseline point, turn each screw in a clockwise direction with a flathead screwdriver until each screw seats, then back both screws out 1-1/2 turns.
Turn the engine on and allow it to idle. Turn each idle mixture screw, one at a time, in a clockwise direction 90 degrees. Continue to turn each screw in 90 degree increments while listening to the engine's idle speed. Once the engine's idle speed has begun to slow, back each idle mixture screw out 90 degrees to complete the process.