Help Please.

I have just tested my drum weapon for the first time and the drill motor im using seems to get hot quite quick.

Ive got a 9.6v old style drill motor (Where there are proper gears that slide on a bar to change the gearing). I have used the original gearing from the drill to get approx 4000rpm(if my calculation are correct). Then using round belt drive to the weapon(see profile).

I am running the robot @ 12v so would be over volting the drill motor. When testing with a 12v battery the motor gets hot reasonably quickly.

Would speed control or some other device help this? Would a different motor be better? If so which type and how much would they cost?
Note: Money is Tight :sad:

Thanks for your help
Glen