Alright, I took a shortcut. That turned into probably a long cut.
Here's what happened. I have another identical Walthers FB-1, so a friend suggested swapping the trucks out to isolate the problem. If I put the trucks from the good FB-1 into the bad FB-1 and it works well, then the problem is in the original trucks. Seemed like a good way to narrow things down, so I did that.
If you look at the PC300006.jpg file, there are two FB-1s in the picture. The Undecorated FB on the left is the original "Bad" one. It was the one running poorly in reverse. The Erie FB on the right was running fine in both directions. So, my plan was, take the front truck out of the Erie and put it in the Undecorated, then test it. The original front truck on the Erie was the one that had the waggle to it, so I figured I'd go one by one, and if just replacing the front truck fixed it, then everything's good and problem solved, right?
Here's where things get confusing.
I took the front truck out of the Erie. And despite the fact that it ran just fine, I noticed that the little plastic yoke at the end of the worm gear was cracked. So... new plan. After that, I took the REAR truck out of the Erie FB and placed that in the FRONT of the Undecorated FB. And since I was now down to just that one working truck, I put the undecorated FB all the way back together, and tested it.
The good news is, the undecorated FB-1 now works just fine. So, obviously the original problem - the "surging" and waggle - was coming from the original front truck.
The bad news is that the Erie is now down to two shot trucks. I'm sure the cracked plastic yoke (see attached "cracked.jpg" photo) is no big deal to fix/replace. But going back to my original concern, I don't see any cracks or other problems with the gears in the original front truck (or either truck, for that matter.)
So, as a possible solution to the original problem, my friend has suggested that the problem may be the fact that there is slack in the worm gear, which allows the gear to move back and forth when you switch directions. See the attached "gear-slack.jpg" image - I put to arrows to show the space my friend says I need to shim out to solve the problem. What is the consensus opinion on whether or not that might be the problem?
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"Don't open the gates!! Who the hell needs a wooden horse that size anyway?!?!"