Hello! I recently acquired a 96 Sportsman 500 from my parents that they have tried for years to get started but couldn't figure it out. So I replaced the stator, cdi box, New coil and plug wire, new plug as well. Still wouldn't start so after pulling the valve cover off and the rocker arms I noticed severe wear on the cam. Then decided to pull the head to see how the valves and piston looked and they were rusted badly. This quad sat for over 6 years after being abused my my younger brother. So I decided to rebuild the entire engine which was only about $500 surprisingly. I'm also an aircraft mechanic so I did my own labor which saved tremendously. So after the rebuild I also replaced the carb, valves, springs and even ported and polished the head then set everything to spec. Started up no problem idles perfect, revs in neutral all the way to full throttle so im thinking great not bad for $500 then comes the issue I can't seem to figure out. It runs great at low speeds alot of torque and power but after 1/2 throttle it starts to sputter and lose power. I have adjusted the new carb over and over replaced the jets and needle moved the e clip in all directions. New air filter new fuel, and the new fuel lines everything is new. Im starting to think it has something to do with the clutch or belt? Has anyone ever seen the same thing when a clutch is going out or a belt is worn. I have not pulled the clutch cover off yet and it has been sitting and probably rusty but wanted to see if this is a possibility before I tear into it. Thank you all for your time.