Hi, can't wait to see what the issue is I would take a top off engine put engine on roller stand and watch operation in forward and reverse I would manually turn flywheel in both directions slowly and see if there is bent side rods or any type of binding then repair and lube what was binding then retest engine, also what is the current draw in forward and then reverse I would suspect the amps higher when trying to run forward then reverse!
p.s. I've seen some motors run smother in one direction then the other sometimes shaft is moving up and down too much play in shaft movement, I recently had a dc motor doing exactly that and had to add a spacer nylon washer to stop shaft movement then motor ran perfectly and start and stop at slow speeds! also sometimes this is due to poor brush wear
Alan
