help me out please

[1Fun400EX]

hey i got a 2001 400ex and it seems to misfire at wide open throttle and kinda seems like its lacking the power it should have.
the things i have done to it are fmf power bomb header, fmf power core 4 pipe, k&n filter, amr rev box stage 3, and i just put a new spark plug in and when ever i have it at wot is sputters
any suggestions as to what it could be?

[BlasterEaten250]

Did you rejet when you put the pipe on?

[1Fun400EX]

i accually bought it with all the stuff on there and this just started happening a few weeks ago

[BigBore24]

yah i think you better start with the jetting. see what main jet it has in it and go from there. also if you have the stock cdi you might just try that since its easy

[n811]

Also check your clutch plates. They seem to lose power on the high gears first.

[dariusld]

Is your air filter dirty?

[1Fun400EX]

air filter is brand new, new spark plug i did find the problem the
guy i bought it from jetted the carb compleatly wrong were the main jet is supposed to be there is pilot jet in there. now my question is what size main jet to put in there i was thinking about a 158, what do u think?

[1Fun400EX]

no its not the clutch plates i know that its not that it looses power on high gears its that it misfires at wot which seeing that there is a pilot jet where the main jet is supposed to be that would deffinatly be a cause

[2quic]

This might sound a bit out there, but, I had a really bad misfire at a hill climb a while ago. At first i thought it was some bad fuel or water in the tank, so drainned the carb, emptied the tank and put some fresh gas in, still misfired. Just as a random i checked the battery terminals, they were real dirty - so a quick scratch up with a screw driver and problem solved. I guess the coil couldn't draw enough current to fire at the top end.

Just a random thought for you

[1Fun400EX]

that is accually true about the battery terminals i have had that problem in the past so they are very clean and tight now