You might have a shorted commutator bar, or one winding in the armature shorted to ground. Every time that one passes under the insulated brush, it will make a big arc. Eventually, the brushes will burn out from the short. If you disassemble the generator and the leading edge of one commutator bar is burned, you've found the problem. It takes an armature "growler" to diagnose a bad armature, something only an electrical service shop or a rebuilder will have now. They used to be standard equipment for virtually all shops, back when generator and starter rebuilding was a routine thing.
Jerry
