I'd rejet the carb, sounds like you are running rich. That is a common problem with these engines, as they are jetted rich from the factory so they will start and run. If you have an NT carb, try a #66 main jet, they come stock with a #70. They take a 5mm dellorto jet, available from Sick Bike Parts, among others. Also, try moving the clip that holds the carb needle to the top groove. (leaner) Mix your fuel at 32:1 ratio, the recommended 16:1 break-in ratio is too much oil. The Chinese don't have access to good 2 stroke oil, so they recommend 16:1 because they use regular motor oil. With a good (expensive) synthetic 2 stroke oil, you can run 50:1, but may have to jet down some more due to the added richness.