I was fortunate with my 401s just clean & oil AND figure out how to work the knobs. I really haven't really messed much with the decorative stitches. I did some straight stitch mending and liked the job it did.
Miriam, I have a 626 that skips stitches, I think, on the left side. I haven't messed with it for a while. I remember I did have to mess with the bobbin area to get it to stitch and get the chain stitch to work.
I am in the south central part of Denver proper. I probably have more questions than answers, but will send a PM.
Janey - Neat people never make the exciting discoveries I do.