Hi - this is my first post, I tried searching for this problem but couldn't find anything.
So I have a 2000 sportsman 500 4x4. I bought it at an auction so I don't have any background, but it wouldn't get over 15mph so I looked on this site and found out about the bad cam shaft, so i replaced that per the manual, so I hope I did it right. Now it runs up to about 45mph, but it takes a long time to get there and it doesn't seem to rev up to full RPM. But here's the really strange part, If I take the Air box and air tube off it runs great, and if I put it in low gear with the air box on it'll hesitate in the mid-RPM range then take off to full RPM, and in Neutral it revs up just fine. I took the carb apart and checked every jet and passage way and they look great. the only weird this is the Jet Needle says "Y237" on it instead of "4HB41-3" like the manual shows. But I couldn't find what "Y237" is? could this be what's causing it? Or is it something else? my next guess would be the clutches, but I've never owned an ATV before so I've never dealt with clutches like this. PLEASE HELP!!!

 

Different # on the jet needle is because someone may have installed an after market carb kit. Taking the air box off usually means the fuel mixture is too rich or air box/filter is restrictive.
This could be because of a worn needle and seat,incorrect float level, wrong main jet size,etc.Main jet should normally be 152.5 to 155. While holding the carb at an angle with float barely touching the float needle, it should measure close to 13 mm / .051 from bottom of the float to the carb body. Since it's an auction bike,I'd just install a new carb kit,make sure the air filter is clean,plus examine the float arm and pin also.If worn and wobbly,the float and pin need to be replaced. Most people over look these two wear items and they are not in any carb kit. Kits are cheap on Ebay,Amazon,plus you can experiment on different needle clip positions and fine tune it. Factory position is the 3rd position from the top.If you still have problems after eliminating the carb,then check into the clutches and belt. Dirt Cheap Yamaha, Honda, Arctic Cat & Polaris OEM Parts & Accessories – Cycle Parts Warehouse

 

So I installed a new carb kit, all the jet sizes were the same in the new kit as what was already in the carb, making sure everything was spotless clean and put back to factory settings. And NO change in performance. Still the same problem, which is really starting to drive me nuts. I don't understand why it works in neutral, low range with just a slight hestiation in the mid RPM range, and it works so well with the air box off in high range. But soon as even put just the tube that goes between the carb and airbox on, it runs like a 50cc not a 500cc.
I also took the cover off the clutches and checked the belt, it measures 1.135 which is worn, but not to the 1.125 "wear limit" and cleaned everything the best I could. I don't have the puller to get the drive clutch off, but did my best to clean it out.
I forgot to say that it says "EBS" on the clutch cover, does that make any difference?

Please, any other suggestions? I can't afford to keep throwing money at this thing or to have to take it into the dealership.

 

Several posts on this same thing about running good with air box off,but not one specific cause. If you're sure the air box snorkel is not restricted and the filter is new or clean,then pull the fuel line from the carb and crank the engine over. Should be a steady flow of fuel.If not a fuel pump could be a problem. You've installed a new carb kit and the problem remains. If the fuel air screw plug has been removed,make sure it's set between 2 to 2.5 turns out from a lightly seated position. You said you replaced the cam(hopefully the exhaust rocker arm also) and said you hope it's right? Did you remove the stator plate to do this and time it from the bottom crank sprocket or just left the chain in place and replaced the cam without removing the stator? Either way you might be off a little on cam timing or stator timing may be retarded also. Ebs clutches have wear items that can affect running such as worn fiber and metal washers in the front clutch,guide buttons,and I've even had some plastic limiter nuts back off causing problems. Item #8.http://www.cyclepartswarehouse.com/f...2000&fveh=5519

 

I made sure there wasn't anything clogging the air, that was the first thing I thought of. Yes, I replaced the rocker arms as well, and I didn't take off the stator plate, I just did what it said in the manual and set the timing marks on the cam gear to the top and the timing mark on the flywheel at TDC, and put the new one in the same way the old one came out. I thought about it maybe being off, but would that let it run better in low gear than in high? and run great with out the air box? Maybe I'll take another look at it and make sure the timing is right with the crank.
I'll check the fuel pump like what you said. I know, I saw a few posts about the same symptoms that I have, but no one posted what fixed it. I'll make sure I post what the cure was so it might help others. And thank you for your help.