Hammerhead Jet, Main Jet for Keihin-Style 125cc / 150cc, GY6 / 250cc Carburetors
The main jet controls the volume of fuel delivery in your carburetor from a little less than 1/2 throttle to wide open.
If you've upgraded your carburetor, air intake, or exhaust, you will find you're running leaner. A larger main jet can help restore the correct air/fuel ratio in your system.
Most stock carburetors come with a main jet between 108 and 115. With a UNI filter upgrade, most systems will need a main jet between 120 and 130, depending on your elevation and vehicle setup. If upgrading the air intake and exhaust, you'll need between 125 and 140 and possibly larger depending on your carburetor, exhaust, and riding location.
You should test various jets until you reach the correct air/fuel ratio when idling and at full throttle.