I don't know why but frequently a problem with a Featherweight can crop up that has never darkened my door before and then I end up with a handful of machines with this same obscure problem. Obtuse and obscure can be fun though, so lets relay the story and see where we end up.
The first of the Featherweights with this particular problem announced itself as a Featherweight that wasn't running very fast. There are Feathers that do run slower than others, but this one was really quite slow. A bit faster than "stop" but not by much.
The First thought was; the belt must be too tight, which is the typical cause. But this machine's belt wasn't too tight. A Featherweight's belt should never be tighter than it has to be to keep the motor pulley from slipping.
The Second thought was; has the motor been lubricated in the last 30 years? It had been.
The Third thought had me gripping the pulley with my fingers and pulling it in and out on it for no real reason. This behaviour is part of my finial set-up/testing of a motor if I had disassembled it to turn the commutator or clean the motor out but I hadn't done that work on this motor, so really I was just killing time while thinking. This is called "fiddling-around" all the time looking terribly busy to the untrained eye.
There was no end play to this particular motor's shaft as I pulled in and out on the pulley, and there should be a little. I removed the pulley and found the motor's shaft had 16 inches of thread neatly wrapped around it that had partially filled the gap between the pulley and the motors housing.The thread was packed tightly enough that the motor could not get up to speed, much less the machine as a whole. The thread got there from a bobbin winding session gone bad. The thread had gotten itself snagged by the motor belt so fast it wasn't noticed. While the thread was dry it let the pulley turn without slowing things down much, but then it begins to wick the motor's lubricating grease out of the motor, which doesn't sound so bad, except that the lubricant makes the thread swell and disc brakes will have been reinvented.
Solution; remove the motor pulley and clean out from behind it. Put the pulley back on.
But note; there are two styles of pulley found on Featherweights, One is made of steel and it has a set screw on the pulley hub that secures it to the motor's shaft. Loosening the set-screw one turn is adequate to remove the pulley.
The second pulley type is a Bakelite plastic that looks like black plastic (unless it was painted tan as found on the tan colored Featherweight). with soft rounded edges in addition to the fact that the pulley is round by function. There is a screw in the hub of this plastic pulley as well, but this screw must be removed completely before the pulley can be removed from the motor shaft. This style of screw threads completely through the motor's shaft. If you only loosen it a single turn as with the set-screw style of pulley the pulley can be broken as you try to pry the "stuck" pulley off the shaft. I am not speaking from experience, of course.
Source: Dave's Blog