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
Last edited by bkay; 08-09-2019 at 06:47 AM.
Reason: clarification