I'm trying to FMQ with a finer thread, Superior 40 wt, and neither of my 301s can hold the tension. I can't switch to another of my machines as all the ones I have in the house are short shank and all my FM feet are slant shank. I did bring my 404 home from church today to see how it does. I've never FMQ with a horizontal bobbin, but at this point, I'm willing to try. Any thoughts on the tension on the 301s? I don't think I can find a repairman locally. Both have been iffy on tension for a while, even while piecing. The 1956 has had the spring replaced by my son, but not recently.

Try a different thread size, maybe a 50# or even a 60#. Have you changed the needle size to fit the thread?

Yes, I've changed everything multiple times. Both machines have FMQ with any thread previously - even rayon embroidery thread. Both machines' tensions have been set as low as they can go just to sew a decent seam, so I think it's time for a new tension spring. I found a shop that will work on them in a town 50 miles away. I may one in for an estimate tomorrow as I have a PT app't there. I'm kind of committed to this thread for this quilt - it's probably 20% done in it, so switching might be a problem. I could switch just the bobbin.

Try to find an Amish or Mennonite shop if you can. These people are adept at working on these old machines.