2001 xc 500, backfires primarly through pto carb. purchased this sled with the problem, thought it would be quick fix, cleaned tank, new fuel, cleaned carbs several times, no luck,pulled jugs found pto carb slightly scored, had both cylinders bored/new pistons and rings, still trouble, pulled and resealed crank case, still trouble, tried swapping coil with my other sled, no luck. I'm ready to try dynomite!!!

What throttle position is it backfiring at? I would suspect a lean condition is what is causing it especially when you had some scoring on the pto side. What do your plugs look like if you run for a while at the throttle position that it backfires at and then kill the engine using the kill button. That may give you an idea if it is lean or not.

Thanks for the reply, it will back fire at any throttle position if held there long enough, hot or cold and mainly out of the pto carb. It will backfire lightly once in a while out of the mag carb. Oddly enough when checking the plugs after running a while does not show a lean condition on either plug. While running I can remove the pto plug wire and the engine will continue to run on the remaining cylinder. While running if I remove the mag side plug wire the engine will run for a few seconds and then quit. Also while running with the air box open I do get a fine mist of fuel coming out of both carbs. At first I assumed this meant bad reeds, I have checked both sides and they look good. I have switched reed cases from side to side and there is no change. I have checked my other sled and it also huffs a little gas mist out the carbs and that sled runs great. I have also raised the needle on the pto carb to try and induce more fuel, this made no change. I thought originally the center crank seal was out and I was losing fuel transfer from side to side. I'm still at a loss! Any other ideas.

Can you reverse the plug wires if they reach to see if the problem is electrical or not?

sounds like a coil issue on the pto side

hey i had the same trouble with my sled and i will gaurante u its your base drain plugs its back firing cause its drawing air . if u look under your exhaust on the bottom of the base there should be two bolt heads tighten them one may even be missing mine was . let me know if it works