Does the motor have some sort of brake mechanism? Motors, especially large motors, have a fair amount of inertia. Once you get it to turn two revolutions, you'll need a way to stop it. You can apply a short pulse of reverse polarity voltage, but that might be inconsistent. Hopefully the large motor has a brake.
What I am going to do with the motor is actually add a bunch of gears to slow it down drastically and increase the torque. When I test the small motor the toy has with 3 AAA batteries and the bigger motor with the 12V battery, they both stop and go at the same speed.