Mikuni carburetor slide carburetors are found on a wide variety of motorcycles. They have a reputation for being easy to tune and have been used by many racers and mechanics for years. They are also very common on vintage street bikes as well as snowmobiles and other gas powered equipment such as generators, welders etc.
These carburetors are typically air cooled and therefore require a richer fuel mixture than water cooled engines. The ideal fuel mixture is 12-15 grams of air for every one gram of gasoline. This fuel mix is optimum for performance but there are other variables that must be considered when tuning or jetting a mikuni carburetor such as engine type, cylinder head size, engine temperature and even the way it is cooled.
Demystifying Performance: A Comprehensive Guide to Understanding Mikuni Carburetors for Motorcycles
The first step in adjusting a mikuni carburetor is to make sure the throttle slide is at the very bottom of the slide passage. This can be done by backing off the idle screw and watching the slide as you turn it counterclockwise. When you see the slide drop, stop turning. Then move the clip up or down on the needle jet – there are usually 5 clip positions on a VM needle and moving the clip raises or lowers the needle in the jet to adjust how high or low it is suspended inside the carburetor venturi.
Next is to check for a leak, make sure the petcock is on and that there is fuel flowing through the carburetor. Now we can start working on the fuel mixture and ignition timing.