Hammerhead Jet, Pilot Jet for Keihin-Style 125cc / 150cc, GY6 / 250cc Carburetors

The pilot jet controls the volume of fuel delivery in your carburetor from idle to approximately 1/2 throttle, when the main jet takes over. 

If you've upgraded your carburetor, air intake, or exhaust, you may find you're running leaner when idling. A larger pilot jet can help restore the correct air/fuel ratio at low speeds. 

Most stock carburetors come with a size 35 pilot jet. Pilot jets should be increased in size in single steps: 35 to a 38, 38 to a 40, etc until you reach the correct air/fuel ratio when idling and at full throttle.

Carb tuning tip:
If your carb is backfiring heavy when you let off on the gas after being at wide open throttle, your pilot jet may be too large. If it's lightly popping when revving down, the pilot jet may be too small. 

When properly jetted, your motor should rev smoothly up from idle to wide open, and back down.

If your motor is popping or bogging when you are on the throttle, usually it is an incorrect main jet size, dirty carburetor or incorrectly adjusted valves.