I have a 87 Quadracer 250. I just got done doing my rebuild. But now the motor cuts out at about 1/2 to 3/4 throttle. I have heard a lot of stuff about the powervalve. Could this be my problem, and how do you fix it. I have put a lot of money into it and dont want to give up on the beast.

 

There is an inspection window on the right side of the motor to see if it is working, the arm should move when you punch the throttle.
It might be a jetting problem. You need to change your neelde clip position. Check out the spark plug after you ride for a while, Sounds like it might be tto lean.