We have taken the 400 apart multiple times to try and find out why oil was coming out of the breather tube, it was orginally taken out in the first place to replace the center gasket. We also put a wiseco 11:1 high comp. piston and a .020 bored cylinder. We started it and it seemed to run fine and we noticed oil coming out of the airbox. Then we took it down and made sure the rings were spaced and the piston wasn't backwards. Put it together and the oil still came out. (keep in mind every time we only put in 1 and a half quarts of oil overfilling oil is not the problem) Then we made sure the oil pump and the lines weren't clogged cleaned and we also took the wiseco out and put in a stock piston and put back in a stock cylinder. It doesn't smoke or nothing just spits oil out the breather tube. The only other thing i can think is wrong is that the gasket is partialy covering the slot for the breather tube.

 

This thread is why i believe that the center gasket was the problem.

blowing larg amount of oil out crank case breather of 400EX? [Archive] - ATV Riders Forum

 

We put the wiseco 11:1 high comp. piston with the 0.020 bore cylinder back in and replaced the center gasket but made sure it was on straight with the sticky gasket stuff applied to the gasket. We started it up and it ran good and no more oil came out of the breather tube.