It's a nice day today, almost 60, so back to this machine. I did the following:
1. cleaned the inside of the race, removed some crud, not the worst I've seen
2. cleaned the feed dogs
3. cleaned the bobbin case including removing the small leaf spring
4. the tension unit was already apart so I reassembled according to the adjusters manual
I tested on a lighter fabric, an old bed sheet (the first test fabric was a lighter weight denim). I also went from a size 16 needle to something a little smaller, not sure what size as it was loose in the catch all needle container. I used different threads, upper and bobbin (I don't know enough about thread sizes to say more than that).
With the straight stitch plate and straight stitch foot, the straight stitch was noticeably better, still not perfect, but it didn't look like a mini zig zag anymore. With the zig zag plate and foot I encountered the same problem, but only had to increase the tension up to 6, instead of 8 before, to get a better looking stitch, so I guess I'm making some improvement. But, if I tried to run with any speed I broke the upper thread, so that's a new problem.
This machine had quite a bit of really cruddy old oil, brown and green, all over it, and I put in quite a bit of elbow grease, so I'm not ready to consign the machine to failure status.
Janey, I am still reviewing the links you sent. Thanks for everyone's input.