If I have to adjust a bottom tension...very rarely, I hold the thread with the bobbin dangling from it. If I give it a quick jerk, I expect it to give me a bit more thread. Not very much. Seems to work pretty well. And yes the bobbin screw needs very small adjustments. 1/4 turn is a big adjustment.

What generally causes those thread nests at the beginning of a seam is not holding your thread tails. An option to holding on to the ends of the thread until the machine has made it's first stitch, is to turn the handwheel until the needle is inserted into the fabric. After the needle is in the fabric, you press the foot pedal.

You don't say which vintage singers you are having the problem with, or where the nests occur (beginning of seam, middle of seam, randomly, all the time, on bottom of the seam or on top of the seam). The above suggestion is for all vintage sewing machines when the thread nest is at the beginning of the seam on the bottom.

If the nest is occurring randomly on the seam line (but sews right sometimes) and it is a drop in bobbin (singer 201, 400 series, 500 series, 66), cleaning the bobbin area meticulously will solve most of the problem. You have to take the bobbin case out and clean under it. Find a youtube video on how to do it on the particular machine you have (involves q-tips, sewing machine oil, cotton balls and toothpicks). The only one I've taken apart and cleaned where it solved the thread nest problem is a 401, but it wasn't hard to do. I took a 201-2 apart for another problem and it was harder to do, but with help from the board, I got it back together.

If all you are getting is a big thread nest that never goes away, then you may have a tension problem. Generally speaking, that would not occur on multiple sewing machines and would be specific to one. And, as others have mentioned, a mistake in threading, bobbin type or needle direction would be a more likely culprit than tension.

Tension adjustment problems are usually evidenced in other ways (barring a missing part). Your manual will describe how to adjust the upper tension.

More information would help narrow the scope of the problem.

bkay

One more thought - what type thread are you using? By that I mean how is it on the spool? My 15-91 does not work well with the criss/cross wound spools, so I had to get a thread stand which brings the thread up over the machine and then I use a bobbin with a hole to thread it thru on the pin on top of the machine and then finish threading the rest of the machine. I came by the idea when my machine was acting that way and I semi remembered the man from Superior Threads had a video of the different spools of thread and how different the thread comes off the spool. I have to do that for metallic thread on my embroidery machine too, not sure why but someone said it helps and it does. So I don't care why.

I use the bobbin on the pin to get the thread back to the level it needs on top. It feeds thru easily - my thread stand is rather tall for my 15-91 which sits in a cabinet.

Hope you find a solution soon.

Look at the bobbin case and see if it is aligned correctly with the needle plate. If the finger is not between the slots bird nest can occur!

Thanks for all the suggestions. I don't have a proper thread stand but use large spools. Since I have the problem with multiple machines, I think that is a good place to start. I'll let you know when I get the problem solved and what worked. Thanks again for the help. Everyone here is so helpful!

try small spools first before you move to the drag of big ones. Hope you get it.