Somethings don't happen often, but when they do happen, they happen in clusters of bunches. A problem was simply stated, "my machine is really running slow" and when we went to the typical culprit, the belt being too tight, we found that this wasn't the problem, this time around. And then the same thing happened three more times within this week.
Checking the belt's tension was the first thing tried. If too tight the machine will slow a lot and it is hard on the motor. The belt is to be no tighter than is needed to keep the belt from slipping at the motor pulley. The belt's not being too tight not being the problem looking elsewhere was in order. Removing the motor's pulley revealed the problem. Wound around the motor's shaft, between the pulley's inner flange and the motor housing, was an easy foot of thread packed in very tightly and dragging the motor speed down dramatically. With the pulley removed the wad of thread pulled off like a miniature donut. I suspect it got there by thread going native while winding a bobbin.
The solution is to remove the pulley and slide the thread donut off the motor's shaft. The trick is to make sure you remove the pulley correctly according to the type of pulley you have on your motor.
There are only two types of pulley; one of metal (steel) and one of plastic (Bakelite). The metal one looks like blackened steel, because it is. It can be attracted to a magnet and has a hole in its hub as it looks at you from the motor with the shaft pointing at your face. The plastic pulley looks like a double brimmed top hat, soft rounded edges on the hub and there is no hole for the sahft extending completely through the pulley's hub as it does on the metal pulley.
The blackened metal pulley has a set-screw that holds it to the motor's shaft as it bears down on a flat spot ground onto the motor's shaft. Loosen the set-screw one turn and the metal pulley can be removed from the motor. Just pull it off the end of the shaft.
The plastic (Bakelite) pulley found on white (pale green) Featherweights and painted tan to match the tan Featherweight's color is also found in natural black on some late model black machines such as the model 222. This pulley has a threaded pin that can be removed from one side of the pulley's hub. The threaded pin runs through one side of the pulley's hub and completely through the motor's shaft. This style of screw-pin has a slot like a screw head for the complete removal of the threaded pin.
The first guess for a machine that is running slow is that the motor belt is too tight, and now you know the second guess.
Source: Dave's Blog