Hi i have a polaris sportsman 500 1997 4x4 4 stroke im having problems when i go to rev it it revs up a little then it back fiers then it gose back down to an idle then it reves back up over and over ive rebilt the carb and its blowing black smoke out the back

 

Wore cam lobe maybe. Polaris Camshaft - The Garage Journal Board

 

Take the valve cover off and look at the cam. left exhjaust lobe is probably worn out, the older ones did this. bet thats it.