RE: sprint 2 sport
You might be over, or under geared. Smoking the comm like that often means there is something else wrong. When there is NO MOTOR in the car, is there a ton of drag when you try to roll the car? You might have some binding.
By now, with all the money you spent on cheap brushed motors(which often don't live very long anyway) you could've bought one decent brushless system and be done with it.