2005 49cc zuma transmission problem?

[jesse]

please help! My little zuma is not reaching top speed and is shifting strange. I have owned 3 separate 49 cc zuma scooters and at the elevation that I live at they would all go 37 mph on flat ground. I recently bought another one "2005 49cc 2stroke" and it worked fine for the first week. Then it it would only reach a 32mph top speed and after 5-10 mins of riding it has acceleration issues. But the acceleration issue only happens after I ride it for 5-10 mins. I am mechanically inclined but for the life of me I can't figure out what the problem is. I pulled the fuel tank and cleaned it out. I removed the carb and thoroughly dissembled and cleaned it- it wasn't dirty. I replaced the roller weights witch were flat sided but the problem did not go away. I installed a new athena drive belt but the acceleration problem got immediately worse so I reinstalled the factory yamaha belt. Any advice would be great! Thanks.......