How to Adjust a Carburetor Idle Mixture

Adjusting your motorcycle's engine idle speed requires adjustment of the carburetor's idle mixture screw. This screw allows air into the carb's pilot system where it mixes with fuel. The position of the screw alters the amount of air allowed into the system, changing the amount of fuel that is added to the mixture. Closing, or tightening, the screw restricts the mixture and creates a leaner condition and lowers the engine's idle RPM. Opening, or loosening, the screw creates a richer mixture to increase the idle speed. Adjusting the screw is a fairly simple process, but takes a little patience.

Things You'll Need

• Small screwdriver, flat-head

Instructions

1. Locate the idle mixture screw on the carburetor body and check the screw position. Using a small flat-head screwdriver, slowly turn the mixture screw clockwise until it is fully seated against the carburetor housing. Take note of the number of turns it took to seat the screw. Using the exact amount of turns, reset the idle screw by turning the screw counter-clockwise.

2. Start the motor and allow it to warm up for five to 10 minutes. This will bring your motorcycle to its operating temperature, providing an accurate baseline to set the idle mixture.

3. Adjust the idle mixture screw with a small flat-head screwdriver, turning the screw a quarter of a turn at a time until the motor is idling at your desired RPM range. To raise the RPM and enrich the mixture, turn the screw out by twisting it counter-clockwise. Lowering the RPM and leaning the mixture is done by turning the screw in, twisting the screw clockwise.

Tips & Warnings

• Obtain a service manual for your motorcycle for in-depth carburetor settings and adjustment techniques.

• If you do not feel comfortable performing this task, seek the help of a qualified technician.

• Do not adjust the idle mixture before warming the motorcycle to its operating temperature to prevent an improper engine idle speed and loss of performance.

• Do not loosen the idle mixture screw beyond three full turns to prevent the screw from falling out of the carburetor.

References

• "The Professional Motorcycle Repair Program;" Professional Career Development Institute; 1995
• Dan's Motorcycle: Carburetor Theory and Tuning
• Mikuni American Corp.: Carburetors

Resources

• Mikuni American Corp.: Mikuni VM Carburetor Super Tuning Manual
• Keihin Racing Carburetors: Jetting Your Slide Valve Carburetors