Depends a little on the vintage of the motor.. - Some are more open at that end of the motor, and you can flush the motor out with a 'dry' contact cleaner..
By 'dry', I mean one that does not leave a residue behind.
Best way is to take the motor apart, BUT.. This may kill, rather then cure, on the odd occasion.
I have found a thick 'paste' of carbon/oil/grease/general-muck in the gaps of the commutator. you can remove this with a blunted cocktail stick. Then a CAREFUL polish with a lint-free cloth, bit of isopropal (or meths). - Don't use cotton buds, they leave 'fluff' and are not environmentally correct these days!
There is (at least) one thread on here, about disassembly of Buhler motors.. I think Neil is the expert on this?