Michael Cahill

I agree with above, try readjusting the timing it and making sure the timing chain tensioner is properly adjusted.

I have a 1998 YZ400f i just recently got a few months ago. It runs awsome and is an all around great bike! but... about ever couple of rides (a hour or two each) i foul a plug! (iv'e gone through 3 now) the plugs are all carbon fouled. when it runs it runs awsome, but you'l go to start it up the next day, and it wont even fire. i have already leaned up the neetle jet a notch and the pilot jet a bit too, and i'm waiting to see what happens. is this all i need to do, or do i need to make furthur adjustments? Thanks, Michael