I have a 2004 500 sportsman that did the same thing.Tore the thing apart,cleaned the carb and numerous other things and come to find out it was the snorkel from the air box goes through a piece of foam under the gas tank fairing had moved ahead and as a result was getting sucked in the end of the snorkel thus restricking the air flow to the carb.I mopulled it back down on the snorkel where it should be and it works the very best.I wish I looked there before doing all those other things.Oh well.live and learn.

 

dropped pilot jet from 40 to 30 and dropped main from 172.5 to 167.
revs 1000 rpm higher and goes 10 mph faster than it ever has. not idleing quite right, may have to go back up on the pilot. ran about an ho so far, going to pull the plug to check colour. the original pilot had no cross holes drilled through it? is this normal or is it a defect from new?

 

My '05 SP400 is doing the same thing when climbing up a good hill hill high range. Not so much in low unless I'm pulling a heavy load. Will do it more with a heavier operator. I understand my main jet to be 172.5 but planning on dropping to 167.5 and see if that works. My plug looks to be OK. Not sure why yours suddenly started acting up. It might be dirt found its way in the carb.

 

I lowered my pilot jet from #40 to #30, lowered main jet from #172.5 to #167. totally different machine. My pilot jet did not have any cross holes in it, new one did? Is this normal? doesn't idle quite right now, may have to go back up with the pilot. more rpm now and faster. This had been an intermittent problem for a year or so now, warmer temps seem to affect. so far so good, hope this is a permanent fix?