I'm sorry, this is a nonsmoking forum please extinguish your motors before entering. (just kidding!!!
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Seriously, with out seeing the motors it's tough to tell if something is truly wrong. It is possible the "smoke" you saw could have been dust that blew out of the motor when it was first turned on but since you mentioned a burnt smell I'll lean away from the dust idea. It is also possible that during assembly at the factory grease could have gotten somewhere it shouldn't have, say near the brushes or electrical contacts, and that is what you saw plus it could also give a burning smell. The final possibility is that something is wrong with the motor or the way you hooked them up. You did mention "motors" which I take as more than one and the possibility of two defective motors in the same kit is unlikely although still possible. Did you do any modifications or disassembly to the motors before running them? Can you see anything burnt or discolored in the motor? You may have to use a flashlight near the motor vents and if there was anything it would most likely be at or near the brushes. My last question would be did you run them anymore and the smoke was only at the initial start up? We haven't had any smoke from our Bosch motors.