i sure appreciate all the ideas! i wonder if the thread itself is the problem. If 30/32 machines had trouble with the YLI . . . .that seems pretty damning of the thread. My machine is really a gem and i've never had problems with any thread breaking before, even if it isn't the perfect FMQ machine. And it was so weird that when I pulled the thread ONLY through the tension disks, the tension seemed fine. Then I threaded it the rest of the way and pulling it through the needle, it seemed too tight. I even tried skipping a thread guide here and there because I'd read that as a suggestion if you had too much tension. That just seemed weird and didn't seem to make a difference.
purplefiend, I only moved the tension to zero because i was trying to reduce the tension because of the thread breaking. Normally I have it around 3 or 4 and it's a good stitch. I had the stitch length regulator to zero.
miriam and spiralt, I did try 2 different brand-new Schmetz 90/14 top-stitch needles.
Just got a call from the guy repairing my machines and they are ready! I got the machines both in Sept/Oct and haven't really played with them yet. Got them cleaned up and they worked, but I was afraid of the electrical. Turns out that was a good thing. He said the 15-91 had to be rewired back to the motor - the wires were rotten from too much oil, or something like that. The wiring for the 66 was ok, though. That 15-91 had been neglected horribly in a garage and when I got it ($35 including its case) it was filthy. Shined up beautifully - I can't wait to try it.
This is the website of the guy that's worked on them - they restore old machines SO beautifully!
http://stagecoachroadsewing.com/
I think I'll try the YLI thread in the 15-91 and see what happens.