Keihin carburetors are manufactured to bolt onto stock motorcycles and watercraft without the need for tuning. Significant engine upgrades such as high-performance pistons, exhaust systems or computer chips may require adjustment of the Keihin's idle screw, float height or three jets. The adjustment process should start with the idle screw, move to the float height and finally working with the pilot, main and needle jet to ensure the idle and acceleration characteristics are smooth and free of sputtering or hesitation.
Start the engine and allow it to warm up for 2 minutes. Turn the idle screw, located on the air cleaner side of the carb, clockwise until it bottoms out. Back it out one quarter turn and listen to the engine's rpm. Continue making quarter turns until the rpm no longer increases --- indicating the idle screw is adjusted properly.
Unscrew the four corner bolts on the bowl and remove it from the carb. Measure the distance from the bottom of the plastic float to the gasket surface on the base of the carb's main body. If the float is not at the proper millimeter measurement as per your carburetor's manual, bend the metal tab on the float arm until the correct measurement is achieved. Leave the bowl off for the moment.
Unscrew the brass main and pilot jets and note the stamped size on each. Increase or decrease the stamp size based on the characteristics of the engine during acceleration. If acceleration is sluggish or sputtering, reduce to the next smaller jet sizes to lean out the fuel mixture. If the engine overheats, seizes often and pings, increase the fuel to air ratio by upping the jet sizes to the next number. Screw the new jet sizes into their respective position and re-install the fuel bowl, screwing the four corner bolts into the base of the carb's main body.
Start the engine and test the throttle from idle through wide-open throttle to ensure the engine runs smoothly at all ranges, indicating you've properly adjusted the carb.
