Keihin CV high RPM carburetors are installed on small displacement, single-piston motorcycle engines. The bowl and float valve assembly controls the amount of fuel delivered to the carburetor bore during second gear full throttle, from 2,500 to 3,000 rpms. When the fuel float level is improperly set, the engine is either flooded with excess fuel or starved for fuel, causing an inaccurate air-fuel mixture within the combustion chamber, resulting in sputtering or stumbling acceleration characteristics.
Start the engine and rev the engine to 3,000 rpms. Listen for either a “wet” rhythmic, soggy sound or a dry, flat sound. This test will indicate whether to raise the float valve tang (when wet) or lower it when the engine sounds dry and flat.
Unscrew the four corner bowl screws at the base of the carburetor body and carefully slide the bowl downward, off the carburetor.
Measure the current height of the float from the gasket rim of the bowl to the highest point of the float with the millimeter measuring gauge -- where the float’s tang touches but does not compress the float valve’s spring. Using this measurement as a base line, bend the tang upward or downward 1 mm based on your engine test diagnosis.
Slip the bowl, with the float inside up against the base of the carburetor body, and re-install the corner screws. Start the engine and run the throttle test again, bringing the engine to 3,000 rpms and listening for a wet or dry sound. If the sound persists, continue making adjustments at 1 mm increments until the engine rpm sound is smooth, warm and crisp.
