The exhaust pipe turning red out of head. Popping back when lettering off throttle. I have cleaned carburetor twice. The air adjust makes no defiance all the way in or all the way out. I cheek the valves and are int 4 thousand ex 6 thousand. Don't know what I'm missing? I bought the quad last week it has had this problem a while because it's melted some plastic around the exhaust pipe. Any help is appreciated. Thanks

 

With the bike being new to you, the previous owner may have done anything to it. Is the air filter or exhaust modified? Jets may not be standard, I would check the valve timing too, it could be a tooth out. If you are sure the carb is OK, I once had a Kawasaki 300 with a cherry red exhaust, and it proved to be a soft exhaust cam, simply wasn't opening the valve long enough to let all the gas out, valve clearance was OK as it wasn't the follower, but the cam lobe that had worn.