I bought a treadle cabinet last week on ebay. It cost $10.50 and the picture of the machine inside was corrupted during upload so it was a mystery. When I picked it up, the cabinet was missing its treadle iron - arggghh! OK, the machine... hmm.. black and gold, apparently in mint condition, full sized. It was an early 201k! This cast iron beast was more than enough to make it worthwhile. I was expecting a model 66 and instead got the best domestic Singer ever made in mint condition.
On getting it home I cleaned it up. Major dust under the feed dogs as usual. Gave it tri-flow all round. Next problem is that of drive. I had a spare singer motor and light in the shed so out they came. The motor was really weak and sparks were lighting the place up like the proverbial Christmas tree. The problem is caused by carbon, oil and dust building up around the armature, where the brushes contact it. Singer motors are made so you can remove the brushes without separating the motor. There are two screws holding two small pieces of bakelite. Take these off and the rectangular brass tubes can be removed. Remove one of these at a time. My mechanic (dad) told me that even putting them back on opposite sides can reduce the performance of the motor. Clean the brush and the armature. I used methylated spirit, which seems to clean it well. When you're happy with it, put the brush back, screw its bakelite cap back on and clean the other brush.
Put it all together and test. Mine was a lot faster and way stronger than before.
Here's the machine because I know you're curious: