[TenGees]

The comm is probably in bad shape, if it's not more serious like open windings or shorted lands. I'm not an expert but on drills I have sanded the commutator. Inspect it and I think you'll find it is damaged (excessive sparking causes damage). You have to be careful not to damage the wires attached to the segments. I've used a woman's emory board because it's fine and flat. Particles removed can go between the landings and could be conductive. A toothbrush and/or air might help. Hold the emory against it and turn it. Routers run way more quickly than drills etc, not sure if this might be too crude of a fix for them but it worked several times for me on slower motors. You only need to clean where the brushes ride.

 

[TenGees]

Thanks for the info gmercer. I would never have guessed that too light of a cord would cause excess heat and never heard of commutator stones. I always assumed the burned comm was from running with worn out brushes / bad springs etc. I was given an old corded Makita drill (nice old blue one with metal belt hook). Before spending money on it, I carved brush 'extenders' out of carpenter pencil lead. With those it ran way better instead of groaning. I replaced the brushes and cleaned the comm as described and it's been running like new for several years now. I always like those old Makitas.