It's been a while!! Anyhow helping a friend out with a 97 sportsman 500. It hasn't ran in many years so I gave it a look-see. Checked for spark.. looked good. Compression was good (can't remember the # off the top of my head but it was within spec). Last time it ran it acted somewhat similar to a 500 that I worked on some years ago with a bad cam, so I checked out the cam. It measured within spec and it looked fine. Took the carb off and bought a rebuild kit (All *****..). It was a little dirty inside from old gas.. but I gave it a VERY thourough cleaning. Checked out the diaphram it looked good. Everything seemed to work smoothly.
It starts and idles perfect. For the most part you can rev it fine too. You can take off very slowly but the issue comes when you give it some heavy throttle. For example, you can punch the throttle from a stop. It was run like a beast for 2-3 seconds then bog and fall on its face. Or you can feather it up to 15-20mph, let off the gas then quickly re-apply heavy throttle and it will pull for 2-3 seconds and then fall on its face. Basically anything sustained over 10% throttle and you've only got 2-3 seconds before it just bogs and quits pulling. No backfire.. no wierd sounds.
Not sure where to go from here. I've tried holding the reverse overide button doesn't make a difference. In neutral you can mostly rev it.. if you keep revving it for a while you can start to get some lazy/bogging revs.
Help?

 

I went back through the carb completely. Verified all the jets and such are "stock" in size. I put everything back together and it just bogs/cuts out on throttle. If I remove the black plastic tube from the carb (essentially disconnect the airbox so to speak) I can get it to run 90% better. If I even just put the tube up against the carb boot (not even insert) it runs like crap again. I've never seen anything like this. I'm not sure why connecting a plain wide open tube to the carb would make it run so bad?