I'm used sometime to phone calls of customers due to tech troubles... will make a good use of this one
Returning to topic, I've been able to restore some motor (not to the old amazing performances, but "honestly reusable"), by cleaning the spaces between the comm contacts (as said by other friends above), BUT I've then placed few drops of cyano glue between comms contact and then laid some sodium bicarbonate on the commutator.
This will become just as hard as ceramic, and when dried required the commutator to be re-trued (not that difficult with just a "Dremel-alike" tool and some thin sandpaper and abrasive paste for final finishing).
This will prevent the carbon particles from sticking in between (and motor will run again, quietly and smoothly)
For sure all this will cost more time than just throw the motor in the trash bin and get a new one, but is a matter of "personal satisfaction" to get 'em running again!