make sure you keep the proper amount of oil in there too. i have seen a few go for just that reason. many 400ex's have a tendency to use a bit of oil when driven hard.

 

Wheelie tip......If you have to continue to accelerate then you are not at the balance point. You just need to go a little higher with your wheelie and keep it there....every time it drops and you have to bring it back up you lose RPM's and eventually you'll run out and be hitting the rev limiter.