Cure 66, My wifes 500 did a simular thing last season. After you check your plugs, compression, carbs , wiring.... check your exhaust pipe and can, Her sled would run fine if rear lifted off the floor but when you put the track on the ground no power! Sometimes critters can get up into your can and pipe and block it off. My wife hit a pole midseason last year and must of broke an baffle off inside the can. If you have same symtoms remove your can , start the sled and see if power returns. also have you checked all of your drive bearings, idler wheels.... Just my 2 cents. Good luck
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