I tried searches because I have seen similar posts, but I can't seem to find one that fits this issue.
2004.5 sportsman, does have over sized tires and a clutch kit. Recently it started sputtering when going down the road. It backfires, bogs down and won't go much over 30 MPH. We have a little road riding at the beginning and ending of our ride and both times it did it, but never at other times of the ride. We could go over 30 at other times though. We have gone this route before after installing the cloutch and not had this issue so I don't think the tires or clutch are the cause. I was thinking jets or a fuel thing. I think it does short bursts of speed on the trails, but once we go for more than a short distance going fast is when it sems to start acting up.
Any ideas guys?????

 

My 04.5 700 EFI did the same thing. Had the plugs changed and it solved the problem. It would idle fine but as soon as I hit the throttle it would sputter and backfire. It started out happening at higher rpm's and at higher speeds, but it got worse quick. Hope this helps.

 

Hey check a # of things. First it sounds like the same problem i have been having and finally fixed her.I run a 04 /600 it wouldn't go over 35 mph boggin and sputtering ,backfire etc.
first we changed Stator,it helped but no results still backfired and won't run at an even throttle.Then we changed the coil.
The coil was bad as well as one wire had been rubbing against frame and and worn through so it was arching out and causing missfire and wouldn't run right.Check to see which electronics are in it,Polaris had a problem with some brand and replaced under warrenty although warrenty had expired. Good luck and let me know if this helped

 

check the plugs... check the fuel filter.... and check your coil

these are probablly the main things that would cause this, remember start cheap, then work your way up