Obviously a number of things can cause an engine to run well in one direction, but not in the other. One thing among several to look for when something electrical in nature is observed, is whether all three armature segment wires are soundly soldered to their respective post. I've had it happen where one or another is ever so slightly loose. When spinning in one direction reasonably good connection gets made, but in the other not so good, and the engine ran sporadically. With all three soldered soundly - smooth running in forward and reverse.
Let us know how it goes and what resolves your issue.
PS 4-pin plug orientation matters on 5-wire AF steam engines. If they are not red-dot aligned, you will get sub-optimal performance. With four-wire engines the plug orientation doesn't matter.
