Mikuni Motorcycle Carburetor Theory 101 Motorcycle carburetors look very complex, but with a little theory, you can tune your bike for maximum performance. All carburetors work under the basic principle of atmospheric pressure. Atmospheric pressure is a powerful force which exerts pressure on everything.

The pilot jet is the part which supplies most of the fuel at low throttle openings. It has a small hole in it which restricts fuel flow though it. Both the pilot air screw and pilot jet affects carburetion from idle to around 1/4 throttle.

The way Mikuni Needle numberings work is:

1. The first number is the length series.
2. The second and sometimes third characters being letters describe the taper from the clip end to the tip in 15 second (1/4th of a degree) increments.
3. The last digit be it one or two characters is the suffix or revision number.

14.7:1
This is 14.7 parts air to one part fuel – an AFR of 14.7:1. 14.7:1 is perfect for idle and light throttle cruising conditions as it’s the most efficient mixture possible, meaning the best fuel economy and lowest emissions.

main jet has no effect on idle. you can remove the main jet completely and it will still idle just like it did.

However it is the dramatic improvement in throttle response that makes the Mikuni flatslide HSR carburetors such outstanding performers. No carburetor made today can match the Mikuni in this regard.

Mikuni engineers work with motorcycle manufacturers to adapt their carbs to a particular design to meet factory specifications. For instance, certain models may need an idle screw on the left side rather than the right to make it more accessible – or maybe the choke lever needs to be operated from the right rather than the left.

The very popular, round slide, VM aftermarket series carbs can be found on vintage street bikes, racing motorcycles, snowmobiles as well as experimental aircraft and farm equipment. I mentioned a third group of Mikuni carbs – but they’re not really Mikuni at all. They are cheap, imitation, Mikuni knockoffs.