Honda Motorcycle Carb Adjustment Tips
by Kyle McBride
Honda motorcycle carburetors deliver a precisely metered air/fuel mixture to the engines. The carburetor automatically adjusts the mix across the entire throttle range. The carburetor adjustments include idle speed, idle circuit mixture, mid-range and high-speed circuit mixture. The idle circuit and idle speed is adjusted with a screw. The mid-range and high-speed mixture is controlled by changing jet size in the carburetor bowl.
Idle Speed
Idle speed is adjusted with the idle speed screw on the side of the carburetor. Start the engine and bring it up to operating temperature. Refer to your specific model's service manual for the appropriate idle speed range. Turn the screw clockwise to speed up or counterclockwise to slow down the engine while observing the tachometer until the target RPM range is achieved.
Idle Mixture
Adjust the idle mix with the engine running and at operating temp. Screw in the idle mixture screw until the bike develops a lean misfire, which will sound like the engine is sneezing back through the carburetor. Once the engine begins to misfire, back the idle mixture screw out just until the misfire stops. This is the optimal idle mixture setting.
Midrange
The midrange is checked by speeding the engine up to about 1/2 throttle. Listen to the engine and observe the exhaust gasses. A lean misfire indicates that the midrange jet is too small and must be increased at least one size. White exhaust gas indicates excessive free hydrocarbons (unburned fuel), and a too-large jet that must be decreased by at least one size.
High-Speed
Once the idle and midrange mixture are dialed in, the high-speed mix may be checked. The high-speed jet is checked while the bike is being driven. Speed the engine up to between 3/4 and full throttle with the bike in first gear. Listen for a lean misfire and observe the exhaust for excessive hydrocarbons. Adjust the high-speed jet size using the same parameters as with midrange--lean misfire, increase the jet one size; white exhaust, decrease the jet one size.
