I have a 2004 sportsman 400, has the exact same problem, replaced carb, cdi, lowered jet size, can't find any pinched hoses. seems to be running very rich, this is an intermitent problem seems when you first start it and take off cold it is fine, within 2 minutes it starts to act up again. there has been lots of this same problem posted on this forum, some one must have found an answer?
I have tried running it without the vent tubes hooked up, no difference !!
Has anyone tried a different brand of spark plug other than ngk, maybe a champion ?
Go to a larger jet, that fixed mine, Can`t tell you what size I just kept drilling it out till it run right. Over the last 18 months it went out of tune 4 times, I went down in jet size each time to get it to run. I then found the air cleaner was no good, I`d cleaned in a few times over those 18 months & it seemed clean. I put a new one in & couldn`t get it to run even with the original jet. So I just kept going up in size till it run. I live in the bush so I can`t run to the parts dealer, so out came the drills.
I hope this helps
My 04.5 SP 500 started doing the same thing as what started this thread, it was like someone flipped a switch, I've gone through and checked everything all the threads have mentioned but nothing has worked. It does run better with the airbox lid off but why would it act up like this so suddenly, there must but something that broke or gave up, jetting wouldn't do that, at least in my situation. Anyboy ever fix this problem>
