Check your battery condition and connections to it, and the cluster of negative wire connections to the frame under the tank l/h side must be clean and firm. You may simply not be getting good current to the shift motor.
I'm pretty sure the OP is talking about the shift motor, when he says "motor", not the engine. And what he's calling coal, is probably carbon residue from the brushes wearing on the commutator ring. If the commutator has some gnarly edges between each contact, it could be wearing the brushes at an accelerated rate.Why on earth is the oil full of coal at such low milage?