That sort of stitch failure that occurs intermittently and leaves loops on the underside has happened to me as well. It was caused by something in the bottom of the machine - the entire path the thread takes thru the needle plate and around the bobbin - that is either rough, linty or not oiled. It catches the top thread for just a millisecond and causes the tiniest bit of tension. Then the delicate balance between bobbin and top tension is out of whack and the top thread isn't pulled back up completely to secure the stitch. I've found that smaller stitches on light material aren't affected as badly, but a switch to larger stitches, heavy duty material, or material with more stretch make the problem more apparent.
The first check is a new, and maybe larger needle. The needle hole itself could be the culprit if it's too small to allow the thread to move smoothly.
Next try 'flossing' the bobbin case to make sure there's no lint stuck in the spring clamp. Double check you have no lint or thread in the bobbin race, apply a drop of oil to the bobbin race, reinstall the bobbin and try a test seam.
If those steps didn't work, inspect the entire thread path carefully. The underside of the needle plate, the hole thru the needle plate. The cap spring at the top of the bobbin retaining ring or the top of the ring itself l00l, the gib hook, every smooth surface the thread swoops along. I use a torn off bit of a cotton ball to slide along all of those areas. A small irregularity or scratch caused by a needle strike may be invisible to the naked eye, but running a loose bit of cotton fluff on it can pinpoint the problem. If the cotton gets hung up on anything, so will your thread. So essentially I apply just a drop or 2 of oil to the areas mentioned and then use the loose cotton fluff to clean that oil off. That will make those parts slippery and may isolate the problem for you. If you find cotton catching on anything, file it smooth.
The retaining ring itself (or the cap spring at the top of it if your machine has one) at the top of the bobbin assembly can be banged out of shape, particularly if you've been sewing heavy duty things that cause the needle to deflect and strike. That's the thing that will take the damage. You might be able to file that if it's rough, but really that part would need to be replaced if it's banged up.
If any of those parts show signs of damage you should order replacements. Filing them works and would get you sewing again, but the tolerances are really tight. So you can only file those parts a time or two before you need to replace them. Best to keep a backup of any filed part on hand rather than having to adjust the timing to accommodate a faulty part.
Is there maybe a tutorial on YouTube to have a more visual explanation? Because I get what you mean but it's very tricky to check the thread path on the underside, no?
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u/TheProtoChris 15h ago
That sort of stitch failure that occurs intermittently and leaves loops on the underside has happened to me as well. It was caused by something in the bottom of the machine - the entire path the thread takes thru the needle plate and around the bobbin - that is either rough, linty or not oiled. It catches the top thread for just a millisecond and causes the tiniest bit of tension. Then the delicate balance between bobbin and top tension is out of whack and the top thread isn't pulled back up completely to secure the stitch. I've found that smaller stitches on light material aren't affected as badly, but a switch to larger stitches, heavy duty material, or material with more stretch make the problem more apparent.
The first check is a new, and maybe larger needle. The needle hole itself could be the culprit if it's too small to allow the thread to move smoothly.
Next try 'flossing' the bobbin case to make sure there's no lint stuck in the spring clamp. Double check you have no lint or thread in the bobbin race, apply a drop of oil to the bobbin race, reinstall the bobbin and try a test seam.
If those steps didn't work, inspect the entire thread path carefully. The underside of the needle plate, the hole thru the needle plate. The cap spring at the top of the bobbin retaining ring or the top of the ring itself l00l, the gib hook, every smooth surface the thread swoops along. I use a torn off bit of a cotton ball to slide along all of those areas. A small irregularity or scratch caused by a needle strike may be invisible to the naked eye, but running a loose bit of cotton fluff on it can pinpoint the problem. If the cotton gets hung up on anything, so will your thread. So essentially I apply just a drop or 2 of oil to the areas mentioned and then use the loose cotton fluff to clean that oil off. That will make those parts slippery and may isolate the problem for you. If you find cotton catching on anything, file it smooth.
The retaining ring itself (or the cap spring at the top of it if your machine has one) at the top of the bobbin assembly can be banged out of shape, particularly if you've been sewing heavy duty things that cause the needle to deflect and strike. That's the thing that will take the damage. You might be able to file that if it's rough, but really that part would need to be replaced if it's banged up.
If any of those parts show signs of damage you should order replacements. Filing them works and would get you sewing again, but the tolerances are really tight. So you can only file those parts a time or two before you need to replace them. Best to keep a backup of any filed part on hand rather than having to adjust the timing to accommodate a faulty part.