I'm not sure where you got your composition formula from but I can tell you from rebuilding several hundred starters, alternators and generators that a) The commutator finish was done on a lathe with a very course finish b) the commutatot was never polished to the finish it should have and c) the brushes are made of some substance which is melting at a lower melting point than what it should run at. If you want to save this motor (???), polish the commutator to a much smoother finish and buy some brushes rated for the current passing through it. You can custim-make brushes if you can find something similar in size. All it takes is a file and then seat them in using sandpaper placed backwards on the commutator. Spin the comm until the brushes take the same shape and away you go. Low amperage = high carbon content, high amperage = high carbon content. Good luck with your motor

 

We used a com stone for seating brushes, used with the motor running if necessary.
http://www.sportsmith.net/faq.aspx?ID=164a8165-e847-41db-b83c-36c983b6fa18
Max.