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), air filter is clean and properly oiled, 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.

 

Maybe when your CDI gets hot it is failing???

 

What does it do on the top end just a smooth die like you hit the killswitch or does do any poping at all?

 

I would check ALL fuel vents, including the tank vent.

 

The fuel vent thing is a very good possibility!

 

id doesn't die it just falls off in power, but there's no noise or popping or any of that. it's just that when it gets hot it's losing power big time

 

so i checked fuel cap vent and everything is good. also, as far as i can tell there's no inline fuel filter unless the petcock has one built into it. i'll check that out but before i tare into it any more how do you feel about the possibility of pickup coil windings expanding under heat and not working properly therefore stator not having correct power output and causing a power loss due to an electrical problem?

 

<div class="FTQUOTE"><begin quote>Originally posted by: live4wheelies

so i checked fuel cap vent and everything is good. also, as far as i can tell there's no inline fuel filter unless the petcock has one built into it. i'll check that out but before i tare into it any more how do you feel about the possibility of pickup coil windings expanding under heat and not working properly therefore stator not having correct power output and causing a power loss due to an electrical problem?</end quote></div>

That can be checked. Ohm out the 3 wires from the stator, on a cold motor, and this might not be convienient, but carry an ohm meter with you, drive til it dies, and quick...check those ohm values again.
On older hondas some of the stators developed cracks, and when they were cold, they conducted correctly, but when the warmed up, the crack would expand, the the magnetism got screwed up. That wont necesarily show up on a ohm check.
Is the fuel line the original s shaped black rubber hose? is it collasped or kinked? When it dies....does it restart right away?