I have a 97 Sportsman 500 that was running horrible, would not hold an idle, when reved up, would not come down from high idle, ran horrible. Replaced carb with one off of a 2010 Sportsman 400 (swapped main Jet) Runs great now until it gets warmed up, then it backfires thru the exahust. If you shut it down and restart it it runs fine for just a bit, then starts backfiring thru exahust again. swapped plug, good fire. Carb throughly cleaned and adjusted. Really need help on this one.

 

check your exhaust cam lobe for wear. lobe height should be 1.29" to 1.27" its common on some of the Fugi 500,455 and 425 using the wrong oil. Do a "search" on the topic on this site. There are many.

Heres 2 of many.

http://www.garagejournal.com/forum/s...d.php?p=624084


http://atvquadsquad.com/eve/forums/a...54/m/658108953
The easiest way to find out if it's the cam is to take off the valve cover and watch the lobes as they go through their travel. The center lobe is the exhaust lobe and it usually is the problem.


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