Is this an atv or something else? Is it 2 stroke or 4, we had a mini quad that had this problem only on flats... Slight incline or downhill and it worked good, had a bent intake turned out from when it got dropped on a down tree.
Nobody noticed it for years till it happened again and the intake snapped. Only after looking at it could you see it would hold a carburetor just wrong enough. Also had a crack in the carb that was not seen and probably sucked air or gas at the wrong time or even acted as a overflow. It was a weird Japanese thing with a metal box and rubber things in it and whatever. Always smelled like it was leaking fuel..