The 400 and 500 series singers are prone to getting oil into the motors from people that over oil the machines.
The oil gets on the commutator and brushes and forms a film from the electric sparking. You can pull the motors out
and disassemble them pretty easily. Use 600 grit emery and polish the commutator and the brush ends to get the glaze off.
I have had some motors that ran real slow with no power and this was the problem.