why do they take them out when you build the motor?? and why did mitsu put them in??

 

they put them in to decrease vibration, if you take em out you will gain ~10hp, but more importantly you have one less belt to worry about, because if the balance shaft belt brakes it takes the timing belt with it

 

that sounds logical enough --tanks for the info

 

It's a comfort thing. They install a balance shaft and belt and it counterbalances the rotation of the crank and camshafts evening out the engine causing it not to shake as much and also extending the life of the motor mounts. If you plan on removing the balance shaft then install stronger motor mounts or you'll feel the vibration increase in the car itself.