This can be carb or even cam related as this year model fell in the years that cams had problems. Check valve clearance(.006 intake and exhaust) and check out the center exhaust lobe on the cam for excessive wear. It should look like the two intake lobes. This manual can help you on trouble shooting from fuel related causes to mechanical and clutch issues and more.1997 Polaris Sportsman 500 Service Manual PDF Download Even a simple disconnecting the black wire from the reverse limiter may solve the problem. They all fail and can cause limiting in forward range also.