i would take the brushes out and polish the commutator to a high gloss with very fine emery paper finest grit you can get, polish commutator until you can see your self like in a mirror, then install new brushes and springs, then it should run light a top, you probably have a lot of arching on the commutator because the brushes are very worn, sometimes even groves worn in to commutator! this solution will fix your motor if this was the problem!
Alan
Alan