accelerate

I just purchesed this bike on craigslist two weeks ago for just $50 bucks. It was a mountain bike w/ a 2 stroke engine kit that was working fine. I used it about three times and now it starts up but it wont accelerate and after about 15-20 sec it turns off on me. Can anybody help please, I...

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...