This is actually an onroad forum. But I will take a shot at it. You didn't give much info. There are alot of possibilitys like slipper clutch, or engine clutch slippping. Air filter plugged, Poorly tuned fuel mixture, Engine worn past usefully life, Engine left with fuel inside for long time, engine is full of corrosion. Throttle linkage is improperly set not opening carb to full power. You didn't say if it starts easy and idles normal or if it blows out unburnt fuel or gets really hot. Your guess is as good as mine.