Very, very likely the issue is the commutator has too much end play allowing it to "walk" upwards towards the brush holder and binding it. The fix is really simple, get at least one (might need 2) 671-23M thrust washer, from somewhere like http://www.ttender.com/partslist.html or http://www.traindoctor.com/index.asp

Judging from what you say, the washer will need to go on the end by the drive gear. You may be able to find such a washer locally, but you need to take on old one to make sure it's not too thick causing binding later. My 681 had a lot of end play causing the same issue you describe. That was the fix.

Carl

 

The problem is a common one with 726/736 and other locomotives that use a similar motor/gear setup. It is caused by wear, running in the forward direction all the time. Usually the problem is worn axle bearings. Look at the rear set of drivers when the loco is running, and you will probably see that they shift to one side and possibly rub the frame. If so, the proper but very involved fix is to replace the axle bearings. A quick fix that many use is to put a thrust washer on the axle to keep it from shifting. Some use a circlip, others have trimmed the flat plastic piece with a slot that is used to keep a loaf of bread closed. If the replacement bearing route is chosen, be sure to replace the worm wheel also, since you have to take it off anyway.

Generally, the armatures on these motors do not have a lot of end play, but you can check yours to see.

Larry

 

And if not any of the above, it could be the brushes too, like you say.
Have you looked at them yet?

 

#736 - Binding Drivers & Shifting Armature Shaft

The solution to the 736 locomotive forward speed issue turned out to be two-fold:
the rear set of drivers was actually binding in one spot in the forward direction only which I thought was pretty weird. But prying the wheels apart a tiny bit on the axle stopped them rubbing on the frame while rotating in a forward direction. Second, the armature was indeed being forced up towards the brush plate when the motor turned in a "forward" direction. Placing a spacer washer on the bottom end of the shaft just above the snap ring and before the existing brass spacer washer stopped that movement completely. The combination of the two changes has made this engine run like a jack rabbit and the owner will be amazed when he first places it back on his layout. No more herky-jerky forward running! I will warn him because, even with the famed Magnatraction, this thing could easily leave the rails and "join the birds" as we used to say on the good ole Big Trees & Pacific. Thanks so much for the much needed advice about what to look for. I had previously serviced the brushes but didn't change the brush springs. They seemed pretty good and are now working well.