ok, i have a 2001 400ex with a stock motor. my problem is that after about 20 minutes or so of riding my quad starts losing power top end and under any good load. riding track, hill climbing, wheelies past 2nd gear,or in sandy areas it just starts dying off pretty bad. i've checked valves a few times, replaced the spark plug (it is getting spark), it has good compression, there's no restrictions in the exhaust flow, pulled the case cover and checked my clutch(shows wear but nothing horrible and i'm replacing that anyways while I have it apart), and also the timing chain timing chain guides and timing chain tensioner all seem okay. someone PLEASE help me. I've asked a bunch of people for any other ideas including mechanics at local shops and nobody can seem to come up with any advice for other possible things to check. again runs ok cold but when it gets good n hot it starts losing power pretty badly. anyone with advice for me it would be greatly appreciated as the riding seaon i now upon us and i want to get out and ride. thank you very much.

 

any intake leaks maybe ????? choke sticking on ????

 

Does it have an in line fuel filter. If its the paper element type, they can cause those isses. The brass screen type filters are more reliable in the gravity fed carbs. Is the gas line the stock 's' shaped one, or has it been replaced? Make sure its not restricted or pinched.
Or it could be the vents are plugged. Next time it starts to stumble....unscrew the gas cap. If that doesnt help, there are two vent lines on the carb. Make sure both arent plugged.