I've run into the "fast-one-way-slow-the-other-way" problem on a
certain 336 that I own. I remember my incredulous joy at finding this
beautiful steamer (with the BIG motor!) for sale at a reasonable
price. I also recall my distress when, after servicing the loco
thoroughly, it ran at only half speed in forward. (It flew down the
track in reverse.)
I know exactly what caused the problem, on my loco, anyway.
I fixed it.
The problem was caused by slightly uneven commutator plates. This seems
to occur occasionally with locos that have been stored away for many years.
What causes it is uncertain, it may have to do with the brushes applying
pressure to the same spots on the commutator for an extended period of
time. Or it may have something to do with shrinkage of the adhesives in
the armature. Or not. At any rate, the
warpage somehow occurs to the same edge of each plate. It isn't
serious enough to be visible to the naked eye, but it has a
pronounced effect. In one direction, there's no problem because the
brushes can easily ride off the "high" edge of one plate and drop to
the next plate. The problem comes when the motor turns the other way.
The edge of the approaching plate is sticking out from the armature slightly
further than the edge of the plate that the brush is already on, causing
the brush to "catch" on the edge of the approaching plate. Both brushes
do this, and so there are six "catches" per revolution of the armature.
That adds up to a significant amount of increased mechanical resistance
in one direction of rotation. (Not to mention increased brush wear.) You
can detect the problem by slowly, gently rotating the armature with your
thumb in the slow direction, while feeling for any roughness when a brush
crosses a commutator gap (you'll need to detach the tender). It will take
a light touch, but the problem can be felt. The armature will turn smoothly
in the other direction.
The cure is to resurface the commutator face. Some here may have
better tools for doing this, but I did mine by using a drill press.
After removing the armature from the motor, the worm shaft was
covered with masking tape for protection so that the shaft could be
inserted into the drill press chuck. Care must be taken that the jaws
of the chuck do not press against the fiber armature sleeve. I cut
hole in a piece of emery ("wet/dry") paper to clear the shaft and oil
slinger of the armature, and set the paper on a flat piece of metal
which was mounted to the drill press table. (The metal piece also had
a hole to clear the armature shaft and slinger. NOTE: The holes must not
be too large. Even pressure must be applied to the entire surface of the
commutator face.) With the DP set at it's slowest speed (710 RPM), and
everything in place, I just lightly pressed the commutator face against
the wet emery paper, checking every 10 seconds or so to see my progress.
After the first "stab" at the thing, the raised spots of the plates were
evident. Nice, clean copper on the same edge of each plate. Turning continued
until the plates were entirely clean and with uniform surfaces. Patience
pays off here. Fortunately, the plates are thick, but it's nice to leave
as much copper on them as possible.
After reassembly, that Northern flies equally fast in both directions.
It runs so well, it doesn't like to stop for Bossy the Bovine. When running
the Northern in her neighborhood, I have to run the throttle noticeably
lower than for other steamers, or things get messy!