No, your machine is not broken – the thread is at fault!
I spent all day yesterday trying to find the culprit of these thread pulls. Surely, the bobbin thread is catching on something! I polished the needle plate, including inside the needle hole. I polished the shuttle, inside and out, the shuttle spring and the cradle. I polished the race. I changed the needle. I changed the foot and polished it underneath. I even polished the tips of the feed dogs! My machine has never been so polished!
I changed the bobbin, that is, I wound a new one using the thread from the old one. And then it dawned on me: the thread!
Look how it is twisting!
Good Lord! What’s with that great big twisted loop?
This is a vibrating shuttle machine which I am using on low tension to sew stretch cotton with a flexible seam. So when the thread is pulled up from the shuttle, the twisted loop simply slips through as if it were a knot! The machine is not bothered by such stuff, so it just continues sewing…
So what would happen in a round bobbin machine that sews with higher tension? You’d think the tension would straighten out the twists, but the result is in fact even worse: it chokes. I tried several machines with different mechanisms – the same effect. Sure, on some it happens less often than on others, but they all suffer from the bobbin suddenly being pulled in the wrong direction, there’s a big crash, and either you break a needle, or get a rat nest on the bottom, or a pull, but something awful inevitably happens. 😦
So here is the culprit:
I don’t really know what happened with this spool of Coats Duet. I use this thread quite often, and never had such problems. But I have another spool in a different colour bought at the same time, and it twists the same way. I’ve also had some spools of Güterman thread do this, but not all. I really don’t understand it… A bad batch?
From now on, I’ll be using either cotton thread, or soft polyester. Cotton thread is usually spun softer, although come to think of it, I did come across one or the other spool with a similar high twist. I found Coats Moon to be a very nice soft spun polyester thread, that is strong enough for regular sewing (much stronger than the usual overlocking thread or the cheap and fluffy polyester thread).
Lesson learned!
Vintage Sewing Machines 
I have found the thread spools that are wound in a criss-cross fashion works better if it lies horizontal. Try a thread stand that feeds the thread straight off the top.
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That’s a good point, Pamela! It does help in some cases, but not with this particular thread. I tried both a horizontal and a vertical holder, and a spool stand – the best way was the traditional one. Also, when you cut this thread, it immediately unravels for up to 5mm from the tip. 😦 Not impressed!
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I remember my 1960s/70s sewing book advising to always check thread for twisting before using it. I thought it was an advice needed 50 years ago, but seems like manufacturers have trouble keeping the tensions inside the thread balanced even to this day! Even on quality threads, it seems.
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That thread was really bad. It must have been a bad batch because I’ve had other thread from that range and it was fine. But now I prefer to buy Coats Moon which is a lovely soft thread that looks great in a stitch and doesn’t give me any issues! 🙂
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Hello Elena
I got my share of twisting problems too. Often it gets worse with thicker thread and especially bonded nylon thread, so many leather workers struggle with it. But I do imagine, that sewing with low tension also can trigger kinks to form.
I tried to make a video about my findings, and especially about how the arrangement of thread guides can help. I made a video about it and discussed it in the Leather workers Forum. https://leatherworker.net/forum/topic/85746-thread-tension-and-twisting/
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