Welcome to the forums. If the centrifugal clutch has been taken off and put back on recently, it could be the one-way bearing but if not, then I doubt it's the problem. It would have had to been taken off and the one-way bearing accidentally put back in backwards. Sometimes, a dirty carb can cause what you described. If you don't think it's the carb, it might be the CDI. I've seen some of the old '89-'89 Fourtrax/Foreman 350s do that and it was caused by a faulty CDI.
Most 4-wheeler problems are caused by a loose nut connecting the handlebars and the seat!!
You only need two tools in life -- WD-40 and duct tape. If it doesn't move and should, use the WD-40. If it shouldn't move and does, use the duct tape.