How to Adjust the Mixture on a Carter AFB

by John Stevens J.D.

The Carter AFB four-barrel carburetor has been among the most popular choices among automotive enthusiasts for decades. Known for its chrome finish, the AFB is also known for its user-friendly adjustment points. Among the most common adjustments are the carburetor’s air/fuel mixture. A mixture which contains too much air will cause the engine to hesitate upon acceleration and potentially destroy the pistons. A mixture which contains too much fuel will decrease the engine’s power. Fortunately, the carburetor need not be disassembled to adjust the mixture.

Step 1

Turn the engine on and allow it to warm up.

Step 2

Locate the two large slothead screws at the bottom of the front of the carburetor. These are the air/fuel mixture screws.

Step 3

Turn one air/fuel mixture screw in a clockwise direction with a slothead screwdriver with the engine idling until the engine speeds up. If the engine instead slows down, turn the screw in a counterclockwise direction until the engine speeds up.

Step 4

Turn the other air/fuel mixture screw in a clockwise direction with a slothead screwdriver until the engine again speeds up. If the engine instead slows down, turn the screw in a counterclockwise direction until the engine speeds up.

Step 5

Adjust the throttle idle screw to the desired idle speed. Because adjusting the air/fuel mixture to its proper setting causes the engine speed to increase, the speed at which the engine idles may have to be lowered. The throttle idle screw is a slothead screw located on the driver’s side of the carburetor. Turn the screw in a counterclockwise direction to slow the engine’s idle speed.

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