I have a 500 sportsman its a 99. I am having a problem with it. I will not take anything over half throttle with the air box on. As soon as I take the air box off it runs great. Can I get some help or advice please. It has a stock jet a uni filter I have tried moving the needle clip to different spots I have adjusted and checked the choke. If any one has an idea I would greatly appreciate a response.

 

Check that the foam that holds the air box inlet tube under the tank isn't blocking air flow plus check that the inlet tube isn't flush against the tank when you sit on the seat blocking air. Had this problem mainly on some Magnums? Plus is the filter clean? They normally run crappy with the air box lid off. You say you checked out the carb,but have you replaced the needle and seat also? Could be any of these? Plus this model was bad about eating up exhaust cam lobes (1996-2000 models) OPT

 

I found that the carb had the wrong needle in it apparently the guy that rebuilt it before I got it put it in there. It has the right one in there now. The filter is clean I even tried my dads brand new filter off his 2012 500. I have adjusted the choke. It is not being blocked off it wont run good with the lid on either with no seat on it. I do not think it is the cam due to the fact that it runs great with no air box. Thats where I am at so far.

 

Have you cleaned the carb and replaced the needle and seat?? You just need to adjust the choke to where there is just a little slack at the lever. If it runs better with the air box lid off then still sounds like too much fuel or somethings blocking air flow. Has anyone tampered with the fuel/sir screw under the carb (has a block off plug) is this plug still in? OPT