I have a RAW 66/80cc motor was running like a champ than around 180 miles into use, the bike does not accelerate past 25. I have tried new spark plugs clutch level and clutch itself appears to be fine. To describe the feeling its as I accelerate I hit 25 feels as if I am pulled to slow down, moves back up pulls me back again and so on.chain feels nice and tight not too much give on it. I have ridin this bike up to 35 mph and now it just feels weak and struggling. Please help much appreciated.