Not all motors can be adjusted for timing. Some are in a 'fixed' position. If yours is adjustable, the base of the motor, where the brushes are, can be turned slightly one way or another. If you advance it, you will get more RPM at the sacrifice of torque. Back it off, you get more starting torque while sacrificing the high end. Go too far in either direction, and it runs poorly and will overheat.
Motors are usually set at the factory for best performance, so you're better off just changing your gearing to get what you want.