I have a bone stock 66/80 cc bees nest on my 26inch mountain bike .Usually I ride at a moderate speed or less just to be nice to the engine . Today I rode a few miles and deciced to pour the coals on and see how fast I could go .At top speed the rpms started to waver slightly up then slightly down then up then down . So I backed off and went to a moderate speed and it ran fine . The screen was clean in the tank, the gasline had no bubbles or air pockets and it never missed a beat while running .I do run my mix a little rich and the plug looked reasonably clean . Could this be a float issue or something else ? One other problem seems to be haunting me lately .Only after the engine gets warmed up ,when I come to a stop and pull the clutch in , it revs up too much. I correct this by turning the screw slightly out .Then the next time I ride the engine is cold and won't idle until I turn that screw slightly back in . I will test for a gasket leak unless you guys think it is related to something else . I have only maybe 50miles on this engine if that and it has run without issue until recently . thanks for any advise John