stumped, Running problem

[underpowered]

i went riding over labor day weekend adn just got my quad out of the trailer. I wawshed it off and then went across the street to play around on it. It ran fine right before i loaded it up labor day to bring it back. Now it cuts out on top end, at first i though i got teh ignition we somewhere, so i let it dry out in the garage overnight. rode it today and it is still cutting out. I am not sure if it is fuel, ignition, or something else. i have seen bikes starve for fuel and do this, lose spark and do this, and i had a 300ex that ran fine one day, and the next did what my 400 is doing now, on it it was a bad cam. mine still has plent of power until it cuts out. sometimes it will cut out at 3/4 throttle, but otehr times it cuts out at 1/4 throttle. Any Ideas on where to start?

[Jared Hamilton]

check your choke if its flipped up it will spudder and 1/4 to 1/2 throttle

[smr]

sounds to me like you have jumped one tooth on your timing. This is a common problem on the 400ex. When they jump one tooth they will have plenty of bottom end but will start cutting out around 1/2 to 3/4 throttle. They usually sound like a rev limiter kicking in..sometimes you will even hear some popping.

This is an easy fix...Just install a chain from the xr400.

good luck.

[underpowered]

the chocke is not on, but i never thought about timing. I will check that tommorow. It does almost sound like the rev limiter.

[kd5sfq]

Check your valve clearances as well.

[underpowered]

the valves were adjusted right before we left. it just happend all of the sudden. The valves should have slowly done it, not all at once. Plus if the valves were to tight or too loose it would not run like it does. I am guessing it is either fuel related or a timing issue. i am gonna clean the carb tommorow to see if that helps. if not i will check the timing and the cam.

[kd5sfq]

Maybe there is a crack in rubber of your intake?

[underpowered]

a crack would cause a lean condition. it it not running lean, rich if anything. the plug is a dark tan or greyish color, right aobut where it should be. i am leaning towards a fuel delivery prblem the more i think about it.

[rYan01400EX]

I also would say it's a timing problem. Off by a tooth like earlier stated. I jumped a tooth on my EX and it did about the same. So I redid all of the timing to make sure I was correct.. Started up, ran fine after that..