Also look for any flash on the worm gear and drive gears on the trucks. Use a gentle hand when removing any that you find with a fine tiny file or a xacto blade. Re lube and break the slower unit in for and hour or so and try mu-ing them again.  If they are still slightly off I like to but the slower unit behind the faster one to buffer any snap the faster unit might cause to the train.

No I didn't know the caps were removable. They seemed like one could bend the cap up but that really didn't isolate it from the plastic yoke that holds the motor. How do you remove the brush caps? That sounds like the solution to my issue. The spring you mentioned, is it the one that makes connection on one side of the frame or are you referring to the two springs that keep tension on the brushes?

Hi all, this my first post to these boards, I hope some one can help me with soldering mobile decoaders to the brush caps of the non-DCC can motors that come with the Bachmann DC F units. I'm pretty decent with my Weller soldering iron. I use low heat, a fine tip, good thin solder, and tin all my leads. Still I'm burning/melting these motors at an unsustainable rate. I this point I feel it's got to be a technique thing that I am missing.  Please help keep my railroad solvent by sharing any techniques I seem to be lacking. If this keeps up I'll be able to run the first dummy unit train MRR history! Thanks