[magnets]

The next step is to remove a CIM from the gearbox and run it like that. If you get the nasty noise, then it's a bad motor. The smell is likely what ever is in the motor heating up. Heating up grease makes quite a smell.


A few more questions
Can you turn the gearbox by hand by spinning the CIM pinion gears?
Are your CIM motors mounted in the right set of mounting holes for the pinions that you're using?

What it sounds like to me is that your center to center some gears are off. This causes a pulsing noise, excess heat, stinky gearboxes, and slow motors.