Carburetor jetting: You must set the carburetor jetting correctly by experiment. Put the main jet in, and run it full throttle long enough to get the plug colored. The best reading on a 4-stroke is a light tan, with perhaps some darkness around the edges. Some factory setting are so lean that they are bright white.
GY6 QMB139 50cc CVK Main Jet Set
Set Fits most Chinese Scooters.
Includes: 5 Main Jets #80 - #85 - #90 - #95 - #100
Easily Tune your Bike for OPTIMUM Power
Start VERY FAT, and then go leaner - to ensure no detonation