How to Adjust the Idle Screws on a Carburetorby Chris Gilliland
Your motorcycle relies on a proper mixture of air and fuel to power its motor. This mixture is governed by the carburetor, which pulls air into a channel called a venturi and mixes with gas before entering into the motor. At a standstill, the air must pass through a small opening between the throttle valve and the venturi, letting a small amount of the mixture into the motor and allowing the motorcycle to idle. At idle, the throttle valve is controlled by the carburetor's idle screw, increasing or lowering idle speed by opening or closing the valve.
Place your motorcycle in neutral and start the motor. Let the motor run for a few minutes to warm up. Once warm, take note of the motorcycle's engine idle speed. Most motorcycles require an idle speed of 1,100 to 1,300 RPM (revolutions per minute) to operate properly at a standstill.
Turn the idle screw slowly using a screwdriver, twisting the screw counter-clockwise to lower the motor's RPM or clockwise to increase RPM. Stop adjusting the screw when the tachometer indicates the desired RPM range.
Twist the throttle open a few times and observe the tachometer needle as the RPMs fade. The needle should return to the desired idle speed. If not, readjust the idle and test it again.
- Some motorcycles are equipped with a knob to adjust the idle speed. Instead of using a screwdriver to adjust the idle, use your fingers.
- For motorcycles that are not equipped with a tachometer, listen to the motor as it idles. The idle should be smooth and consistent. If the motor is stumbling, increase the idle speed until it smooths out. If the motor is racing or sounds very fast, decrease the idle speed until the motor runs smoothly.
- Refer to your owner's manual for specific details regarding recommended idle speeds and adjustment methods.
Items you will need
- Screwdriver, flat-head
- "The Professional Motorcycle Repair Program;" The Professional Career Development Institute; 2000
- Dan's Motorcycle: Carburetor Theory and Tuning
- "2004 Kawasaki Ninja 500 Owners Manual"; Kawasaki Heavy Industries; 2004
- Jupiterimages/Comstock/Getty Images