<!-- from fixya rte -->What causes the bobbin case thread ball up - Singer 1120 Mechanical Sewing Machine

SOURCE: tangled bobbin thread

Okay. I had this problem too. try taking the bobbin casing out. I had a bit of thread lintg caught in a remote area of the under-plate area. Blow it out with canned air and oil your machine. That should help. It fixed mine.

SOURCE: bobbin case and threading singer 6423

This machine has a black plastic bobbin case. You drop the bobbin down into the case with the thread coming off in a clockwise direction. If you look close you will see a slot in the bobbin case where there is the metal pieces. The thread will slide into that and as you pull on it it will work its way through and come out on the other end of the metal. Lay the thread over top of the bobbin case and turn the handwheel to pick up the thread. (make sure you close the slide plate)

SOURCE: Singer Sewing Machine model 9224. Bobbin thread

If you turn the handwheel, can you see a metal ring rotating around the outer edge of the bobbin case? If not rotating, broken gear or timing belt off.
If that ring is turning, remove the needle plate and bobbin case. As this ring now rotates, you will a finger on the ring that will pass behind the needle from right to left and it should catch the thread from the eye of the needle. If this finger, the "hook" passes above the eye, you have a timing problem.

Let me know.

SOURCE: thread balls up in the bobbin/singer sewing machine, also thread

The balance stitch control on this model is below the stitch patterns; to the far right.

If you need a manual you can download a free one at the Singer Company website.

SOURCE: The bobbin case on my Singer Inspiration 4210

From what I gather on other sites, your thread has to be threaded under tension.... You have to make sure you hear a click when your bringing your thread down to the bottom of the "arm". Also, I thought the bobbin case was rather loosely held in its place, I adjusted this by unscrewing the black plate and moving it over just a smidge. The bobbin is supposed to have some wiggle room, but it kept popping up and making the tiny adjustment seems to have done the trick....for now...