Do not modify the motor to try and fix the problem. Find out the reason for the problem and then repair it. If the motor field is loose, tighten it if possible, or bring it to a repair facility. Check that the axle worm gear is not worn, and that the lower armature bearing is not worn either. Replace the brushplate with part #8010-139 if necessary to make sure you have a good upper armature bearing. See http://www.lionel.com/media/supplements/Supplement1to9/1Complete.pdf page 381 for the motor truck breakdown for that locomotive.

Larry