Sounds like the commutator face is touching the rear bush tubes. Can't remember now but when going forward the armature is pushing forward but when going in reverse the armature pushes to the rear. Have a look to see if this is the case. Again can't remember at moment but if you have the wrong armature in it this will cause the problem. I think the armature on this has a brass oil slinger that touches the rear bush/bearing and still gives clearance for the commutator to brush tubes.