The 401A is an excellent machine. One of Singers best ZZ machines. All but indestructible and capable of sewing most any type sewing a non-industrial machine will be called on to do.
They are oil hogs though and have several areas that will gum up solid if not routinely cleaned and oiled. The ZZ linkage under the nose, the cam stack controls, the plunger directly under the flapper to the left of the cam stack.
If it's been sitting for a long time, clean it good internally. I had to use denatured alcohol for most of the needed cleaning on ours, and in some places Hoppe's #9 gun cleaning solvent.
There are 5 (I think) places that take grease, then everything else that moves or rubs against something else needs oil.
Hopefully you will get the manual and slide plate, the manual is needed, but both can be found on line.
Joe