Air leak!
Stop running the engine don't ride it! or serious damage will occur!
what mods have you done! have you done the T.O.R.S. delete?
Spray soapy water or starting fluid around every gasket, rubber boots etc. if it revs up theirs a leaking gasket, or if the spoay water bobbles up.
common place for air leaks is the rubber boot the connects the carb to the engine.
1 Check the rubber boots for cracks and then spray it.
2 spray the carb also. around the top for leaking seals
3 spray base gasket, head gasket etc.
4 dont forget the yamabond gasket that you have to make fot the cases.
5 spray around were the pipe is. if air is leaking from the exhasut it still can blow up.