It could easily be in need of more oil and grease. The gear driven motor might need a bit of tending too, as you know well, old grease can dry and gum up parts. There's a worm gear behind the hand wheel, and luckily lots of good information on this are available. Much the same for the parts taking oil too, keep on oiling and test sewing. The 15-91 is nice, it's many peoples favorite straight stitcher. It should take a general 15 case, a new replacement part should work. Make sure the tiny finger is in the "right place", they come in both 11 o'clock and 1 o'clock position, I'm not sure what your machine takes. The controllers might need some work, but it's usually about checking wiring and adjusting for speed; not slugishness in the movment. There are a few blog posts on how to service the old button bakelite controllers, they are quite good when sorted out. To sum it up; more of the same as mentioned before, but it's always is a bit different from machine to machine.
Last edited by Mickey2; 12-30-2018 at 04:24 PM.