I had a slow running 671 once. Like you, I went through all the normal maintenance but could not correct the problem. The problem turned out to be that the commutator slots at the outboard ends had been packed with a carbon- oil mixture. This was conductive and was shorting the commutator bars. This got so hot, I could see red heat while running in the dark, that it carbonized the Bakelite plastic that the commutator was made of. Once the Bakelite turns to carbon the problem just continues to grow. Apparently the carbon had enough resistance to allow the motor to continue to run, but slowly, but low enough resistance to creat a lot of heat. Once I was on to what was happening, the problem was apparent. There was a small round circle at the end of the slot. Working on it with a dental pick, I could pick out the crumbly carbon chunks.
The fix was to change the commutator. This also required changing the felt pad under the commutator, as it was oil soaked. Since then I have run across this same problem several times and changed several more commutators. The commutator is pressed on the armature shaft, which is slightly knurled. Do not start this repair until you have a replacement commutator in hand. Many commutators are either burned or cracked at the ends of the slots, and they are likely to break when being removed. The two things to watch when replacing a commutator is that the slots go back in the exact same location with respect to the armature coils and that the commutator gets pressed down to the proper location so it works properly with the brush plate. Most post war commutators started with the 226E-?? Brush plate. There are a few where the commutator segments are bonded to something like mica or plastic sheet, but I like the molded Bakelite commutators. I have seen the 226E commutator with a 2333-?? number, but I can see no difference in them. Last time I checked Jeff Kane was selling post war Bakelite commutators.
