I won't claim to be an expert on Mantua steam locomotives, but OTOH, I've been working with them for about the last 35 years or so, so I may be able to offer some suggestions. From what you said about running better in reverse than forward is symptomatic of "cogging" between the bull gear on the main driver and the worm on the motor shaft. Most likely your locomotives have open-frame motors held to the frame by a single screw, and the worm engages the bull gear directly. (Older ones had a gear box and the motor shaft was connected to the shaft that stuck out of the back of the gearbox by flexible tubing or a universal joint. Those ran much better, but were more expensive to manufacture, so Mantua/Tyco dropped them in favor of the direct drive.) To analyze your problem, you will need to remove the boiler (which you will have to do anyway to clean the commutator on the motor). If you don't already have them, get a pair of alligator clips on the end of wires coming from a power pack. Clip one wire to the frame and one to the tender frame. Turn on the power, using the speed control knob, SLOWLY, with the reverse switch in the reverse position (drivers going in reverse). Watch the position of the armature of the motor (the part that turns) relative to the frame of the motor. The end of the armature will probably be back against the brass frame piece which has a bronze thrust bearing in it. Now, throw the reverse switch on the power pack to the forward position. Watch the armature. It will move longitudinally in the other direction until it hits the other thrust bearing. If this amount of travel between reverse and forward is excessive, the teeth on the worm gear will probably engage excessively with the teeth on the bull gear. There are a couple of ways to handle this:
The simplest is to loosen the screw that holds the motor to the frame and see if you can move the entire motor until the engagement decreases. Tighten the mounting screw, but don't overdo the change in position. Also, when you re-tighten the screw, it may tend to pull the engagement back into too tight a position. You can prevent this by placing a thin shim under the front end of the motor. Sometimes just a piece of notebook paper will be sufficient. It will be trial and error, but should make your loco run much smoother in the forward direction. You might also use a piece of brass shim.
Now, if the above doesn't work, try removing the motor altogether and pushing the locomotive forward and back. If it still feels tight running forward, you may have something binding in the running gear. Look to see if any of your drivers are wobbling. It may be that one of your drivers are out of quarter with the others. These problems are easier to fix than to discribe, but re-quartering drivers requires a tool to do it. Since your problem appears to happen in only one direction, I'd look at the motor gear engagement first.
Ideally, the best thing to do with Mantua steamers is to replace the motor with a can motor and install a gearbox. Yardbird used to have some, but I think I bought the last one! Another possibility was the sheet-metal gear boxes from Herold Mellor. Unfortunately, he has been long gone from this mortal coil. Even with his boxes, the original motors needed to be disassembed and the thrust bearings and washers cannibalized to be used with the sheet-metal boxes.
But, I'd bet you will do okay by adjusting the engagement of the worm and bull gear. How many locomotives are you dealing with? Let us know of your progress.
