My wife's sled is a 2004 Polaris XCSP 500. It still runs buts purrs intead of rumbling like normal. It is very slow to go but still has top end speed. Smelled gas, investigated and saw gas leaking from air box. The gas seems to be backing up through one of the two vent lines to the air box. Any suggestions for what might cause this?

a reed valve could be cracked off since you have diff. sound and low acceleration.
You probably already cleaned the exhaust valves.