Fine-tuning your Mikuni Constant Velocity (CV) carburettor-equipped motorcycle is a balancing act of maintaining the right fuel level versus the size of the carburettors' various jets. Although these jets are the primary source of power from a quarter to full throttle, fuel level can greatly affect performance at low speed, cruising speeds and at full throttle. Adjustments to the fuel level are made by adjusting the height of the carburettor's internal floats, which regulate the amount of fuel that is allowed into the carburettor. You must be completely comfortable with removing and disassembling your carburettors to perform this task.
Turn your motorcycle's fuel valve to the "Off" position and pull the fuel lines off of the carburettor. Disconnect the carburettor from the motorcycle's air box and motor. Pull the carburettor out of the motorcycle and remove the throttle cables.
Unscrew the drain screw on the bottom of the carburettor's float bowl with a flat screwdriver. Drain the fuel contained within the float bowl into a suitable container. Tighten the drain screw, then remove the float bowl from the carburettor with a Phillips screwdriver to access the float.
Flip the carburettor over with the float facing up. The tang at the centre of the float should be resting on top of the float valve without compressing the spring at the end of the valve. If the valve is compressed in this position, tilt the carburettor on its side to relieve some of the pressure generated by the weight of the float.
Remove the gasket from the carburettor's float bowl mating surface. Measure the distance between the edge of the mating surface to the top of the float, using a vernier caliper gauge or a float height gauge. Compare this measurement to the float height specifications found in your motorcycle's service manual.
Adjust the float height and fuel level by bending the metal tang at the centre of the float with a pair of needle-nose pliers. Bend the tang down -- away from the float valve -- to lower the float height and increase the fuel level. Bend the tang up and closer to the float valve to raise the float height and decrease the fuel level. Measure the float height again and make any necessary adjustments until the float height is set to float height specified by your service manual.
Reattach the float bowl with a Phillips screwdriver. Reinstall the carburettor onto your motorcycle, following the reverse order of removal.
You can purchase a service manual specific to your motorcycle through your local dealership. Float height is measured in reverse. A higher float height will decrease fuel level, while fuel levels are increase with a lower float height. Small adjustments to the float height, made in 1mm increments above or below the factory specifications, can improve your motorcycle's performance under certain situations.