You may not know what jet size would be a good starting point for your combination.

Why buy a bunch of main jets based on a guess when you may not ever use them again By drilling out your stock main jet to larger sizes you will be able to narrow down what jet size your carburetor needs.

The brass main jets are easy to drill so you can get by with one of these inexpensive drill bit sets.

If your stock main jet is a 100 (stock size in a 22mm Mikuni) you will find this jet size falls between the 62 60 drill bit.

If its lean and you need to increase the size of your main jet, drill the jet out with the next size larger drill bit, which in this case is the 60 bit.

Run the engine again with the drilled out main jet and take another plug reading.

See our Carburetor Tuning section to learn more about plug readings.

Do not skip drill bit sizes because once you drill the main jet too large, you cannot go back to a smaller size.

You do not want the drill bit to enter the main jet at an angle because it could create an oblong hole in your jet.

This means we have drilled our stock main jet to the equivalent of a 119 main jet.

Mikuni Carb Jet Sizing Chart