check twofiftyAZU s advice of having a look at the slip rings if the carbon is sitting on a ridge of plastic it wont be sitting on the copper ring and making electrical contact, likewise the old worn carbon may have bottomed out with that tag and the resulting arking may have caused problems with the slip rings.

simply spin the armature and polish the slip rings with fine wet and dry till they are shiney copper and remove any plastic in that area

i use a pillar drill with the small end of the armature in the chuck and the other in a spare ball race in the vice slow speed, dress the plastic with a broken hacksaw or anything else pointy and then a polish with wet and dry held over a finger
H&S warning!
if you cant do the above without hurting yourself then its natures way of keeping you out the gene pool-you wont die, but you will struggle to find a breeding partner down the disco/chipshop with a mutilated hand.