I have seen a bunch of people post about this same exact issue but have not seen a response that has corrected it for anyone. Please help currently i have a shop rag stuffed in the air box snorkel to make this thing run great. I recently acquired a 99 sportsman, long story short installed a new cam after i had put on a new carb from ebay. Now the thing runs great so long as i restrict the air flow entering the air box and/ or remove the air box completely, if I unrestrict the air flow the thing spits and sputters and loses power. I saw you touch on this in an old post and was hoping you could lead me down the right path. What could cause this? Is the carb running to rich/ lean (I'm not great with carburetors but give me an electric forklift or anything else mechanical and I got no problem). The air box is not in the greatest condition there is a crack by the one mounting ear and the other is missing entirely, I removed the foam seals that were all stretched out and not accomplishing anything and put silicon on the mounting flanges for the snorkel and the output tube to the carb, was thinking about ordering a new seal for the top of the box but with the bottom the way it is it wouldn't matter anyway

 

they are meant to run with the air box completely intact or the results are what you are describing. this is of course with a stock set up bike/carb. usually if it runs better by restricting the air flow then it would seem not to be getting the correct amount of fuel. did you check to make sure the new carb had the same set up that the old one did and set the same way? did you check the spark plug to see it looks like to be running rick/lean? also assuming you adjusted the valves as well.

 

 

Ok so hopefully this thread pops up when people go to search for a Polaris running great with the air box off or restricted air flow through the snorkel when the air box is on! This was after carb and camshaft replacement. I raised the e-clip on the jet needle to create a leaner condition. This corrected the problem for the most part, i think i will have to go back and readjust the valves shortly. But if you have just spent your time and effort rebuilding your quad and it runs like hell you need to tune your carb back in. The manual only gives you base settings you will probably have to adjust from there. Had a hell of a time finding an answer to this question.