I cured DC "jackrabbit" starts in a 2 rail China drive Atlas F7 by pulling the bushing on the worm in the truck and polishing/opening it up a few thousandths. From the factory tolerances were way too tight. There was uneven wear on the worm where it runs on the bushing. I could also feel resistance spinning the flywheel by hand. Before polishing I did an experiment and simply removed the bushings....immediate improvement.
In DC the engine now starts rolling at 1.5-1.6v, before is was 2.2-2.4v and was not smooth...at all.
To be honest, I don't know that those bushings are even needed, IMO you'll loose traction on the rail WAY before you generate enough torque to bend the motor shaft and/or damage the worm.
