If your motor isn't getting warm then your pobably under geared. The motor should be at around 130 degrees after a four minute run. If you gear the motor proporly and it is still to slow and your rebuilding it proporly, then check your speed control to make shure it is set proporly. If the anacer to all of these is yes, then try a diferent motor. Or atleast try to find someone that can test the motor for you, eather on a dyno or in there truck.
Remember, Every thing I post is just my opinion, But if you don't agree with it, You know where you can go.... LOL