My friend has a 03 or 04 Sportman 500 HO that can only go about 10 mph and then backfires and he loses power. The engine doesn't stall, but then any rpm's after idle it back fires when you press the gas and still little power. After about 10 times of pressing the gas and getting backfires we can ride for a few minutes at full throttle and then this horrible cycle repeats itself. Any thoughts on what the issue is? The air box was clean, so I'm thinking it could be the fuel filter.

 

pull the spark plug and check the color if real white its running very lean.