wont run past half throttle????

[dirtking]

i have a 1999 400ex with an hmf slippon, jetted, and K&N air filter, but the thing is it will start perfect and idle but when you go out and ride it will run good through 1/2 throttle but once you get past half throttle it just falls on its face. whats the problem?????

[KISSmyACE]

did any of your lovely friends adjust your throttle stop screw? lol

[jasonwayne222]

clean your main jet, probly the only problem.