A Mikuni carburettor is preset at the factory to work with your engine displacement. This means the main jet and the accelerator jets are pre-installed. To set a Mikuni carburettor you need to adjust two different screws. Both screws adjust the idle/air mixture on the carburettor to ensure your motorcycle idles are the correct RPM.
Turn the engine on and allow it to warm up to proper operating temperature. Keep the engine running through the adjustment process.
Locate the idler screw connected to a black rubber hose on the left side of the carburettor housing. The screw is silver relatively easy to locate since it secures to the on the end of a five-inch black hose.
Turn the idler screw all the way counterclockwise and then turn it in clockwise half way. This will affect the idle of your engine. However, this gives you the ability to quickly raise or lower the idle of your motorcycle at a later time given the temperature outside.
Locate the air/fuel mixture screw threaded into the left side of the carburettor housing. The screw is made of brass and requires a flathead screwdriver for adjustment.
Rotate the screw until your engine idles between 1,700 RPM and 1,900 RPM. This sets your engine at the correct rpm idle when the motorcycle runs at temperature.