Originally Posted by
bearisgray
Why should one thread a machine with the presser foot up? What happens if the presser foot is down instead of up?
If the presser foot is down, the tensioner is engaged and the disks in the tensioner are pressed together, preventing the thread from seating properly. When you raise the presser foot to put fabric under it, the thread may or may not seat itself. If it doesn't, you will have at least an inch or two of big loops on the bobbin side of the seam before the thread works itself in between the disks. At worst, the thread never will seat properly and the whole seam will be loopy and loose on the bobbin side - there's not enough tension being applied to it because it's not being pressed between the disks.
When I've turned the tension adjustment screw on the bobbin holder, I've found that even quarter turns can make a noticeable difference in the stitch appearance.
Yes, once you get the screw into the right neighborhood, even very small adjustments are effective.
I have a Singer 237 (I think early 60s lower end machine) and the tension changes all on its own - so I have to tighten it up every so often.
It sounds like the screw that holds the tensioner might have loosened. I'm not familiar with 237's at all, but look all around the outside of the tensioner and see if there's a tiny screw in there anywhere. If you find one, tighten it up and see what happens.