How to Adjust Idle Mixture Screws

The idle-mixture screws in a vehicle's carburetor determines how much air is mixed with the fuel being drawn through the carburetor while the engine is idling. If the idle-mixture screws are improperly adjusted, it can cause rough idle, stalling and loading up (flooding) while stopped at a light or stumbling when accelerating away from a stop. With the proper adjustment, the transition from closed throttle idling to part or full throttle acceleration should be smooth and even.

Things You'll Need

  • Idle-mixture-adjusting tool
  • Screw the idle-mixture screws all the way into the carburetor until they are lightly seated. Unscrew the mixture screws one and one half turns.

  • Start the engine and allow it to run until it reaches operating temperature on the temperature gauge. If the vehicle is not equipped with a gauge, operating temperature can be identified when the upper radiator hose becomes hot, indicating the thermostat is open.

  • Slowly turn one idle-mixture screw into the carburetor while observing engine idle. Stop when the engine begins to idle roughly.

  • Unscrew the idle screw, while counting the number of turns it is unscrewed, until the engine again begins to idle roughly. Screw the idle mixture screw into the carburetor one half the number of turns it was unscrewed. Repeat the procedure for the remaining idle-mixture screw.