i have a 03 400ex it runs good at idle and mid range rpm but at high rpm it starts missing i pulled the plug and it was dark black and dry the olny mods i have are is slip on pipe k&n removed the choke and two holes in the air box

 

if you got the quad with oil in it drain the oil and put new oil in it

 

i just did a oil change not to long ago and the spark plug

 

You state that it only misses at hight RPM, either you are hitting the rev limiter or because of your mods, you need to rejet. Im not sure what web1666 was getting at with the oil change, but that wouldnt affect your running.

 

thanks for all the help ended up being the outer wear on the k&n fillter was really dirty