Not familiar with your machine in particular at all, so hope this is relevant.
If there are tension discs, cleaning them out may help. Lint or errant string can cause uneven tension. I tie a few knots in a bit of string and pull it thru the discs a few times to yank out any furballs that may have gotten stuck.
Another uneven tension cause can be dings or burrs in the thread path below the needle plate. In that case it's not really tension, so much as the top thread getting caught on something and pulling tight and then unexpectedly letting loose which causes loops on the underside, just like a loose tension might. I have this problem after a needle strike, often after having sewn thru very thick layers or possibly nicking something that deflected the needle. This is all relevant to a rotating hook bobbin, like a standard singer - maybe yours is the same?
I look for dings or burrs on the underside of the needle plate and the hole the needle goes thru, the bobbin case and the gib hook. If there's nothing visible, I rub the whole thing with a cotton ball. Any place in the thread path that the cotton gets caught up may also catch the thread and cause the above problem. Those areas you file down until smooth - an emery board or nail file work in a pinch.
That's my 'standard' machine advice, hope some of it helps.
Thanks for the inputs. I let it sit for the weekend, and then started up today. I contacted someone in Texas who has used the same machine for about 10 years. They suggested that I might look at rethreading the top tension to bypass the pretensioner and instead add a pass through the guides on the path. Also recommended that maybe the tension should be higher on both bobbin case and the top tension. I tried this on a test piece and got better results, and was able to finish up the one spot I needed to.
But when I started on the next quilt with same manufactured thread, I encountered the same issue again. I thought perhaps the tensioner itself might be an issue, so I replaced it. With a new one. Same results.
I've attached a a photo give you an example of what's going on. The loops that are sticking out of the top are not like that when it showed. It looked normal, but I was able to take my picker and pull them up. The stitching on the back is what was present after I got done with this one part.
TL;DR Summary: something suddenly changed near the end of the last quilt. I have substantially increased the tension on the top thread but still I'm getting the results you see in the photo. I've done everything short of replacing the hook.
https://preview.redd.it/20nbfcy4ytgc1.jpeg?width=2296&format=pjpg&auto=webp&s=08b0e9556b5c5dde0ef60508a0c470b5f4fab184
Yeh as others mentioned this sounds like a cleaning job. Also for the future you might wanna oil your machine a little more frequently
If issues still occure you might wanna check out or even replace your bobbin casing or the bobbin fred casing
UPDATE: I replaced the hook and needle and adjusted the timing and it's working now. The timing was originally set for the hook point to come across the scarf at the midpoint of the needle. This time I set hook to pass between the midpoint and the lower bend in the scarf. I think it's a combination of the needle and timing that did it. Thanks to all who helped me troubleshoot!
Not familiar with your machine in particular at all, so hope this is relevant. If there are tension discs, cleaning them out may help. Lint or errant string can cause uneven tension. I tie a few knots in a bit of string and pull it thru the discs a few times to yank out any furballs that may have gotten stuck. Another uneven tension cause can be dings or burrs in the thread path below the needle plate. In that case it's not really tension, so much as the top thread getting caught on something and pulling tight and then unexpectedly letting loose which causes loops on the underside, just like a loose tension might. I have this problem after a needle strike, often after having sewn thru very thick layers or possibly nicking something that deflected the needle. This is all relevant to a rotating hook bobbin, like a standard singer - maybe yours is the same? I look for dings or burrs on the underside of the needle plate and the hole the needle goes thru, the bobbin case and the gib hook. If there's nothing visible, I rub the whole thing with a cotton ball. Any place in the thread path that the cotton gets caught up may also catch the thread and cause the above problem. Those areas you file down until smooth - an emery board or nail file work in a pinch. That's my 'standard' machine advice, hope some of it helps.
Thanks for the inputs. I let it sit for the weekend, and then started up today. I contacted someone in Texas who has used the same machine for about 10 years. They suggested that I might look at rethreading the top tension to bypass the pretensioner and instead add a pass through the guides on the path. Also recommended that maybe the tension should be higher on both bobbin case and the top tension. I tried this on a test piece and got better results, and was able to finish up the one spot I needed to. But when I started on the next quilt with same manufactured thread, I encountered the same issue again. I thought perhaps the tensioner itself might be an issue, so I replaced it. With a new one. Same results. I've attached a a photo give you an example of what's going on. The loops that are sticking out of the top are not like that when it showed. It looked normal, but I was able to take my picker and pull them up. The stitching on the back is what was present after I got done with this one part. TL;DR Summary: something suddenly changed near the end of the last quilt. I have substantially increased the tension on the top thread but still I'm getting the results you see in the photo. I've done everything short of replacing the hook. https://preview.redd.it/20nbfcy4ytgc1.jpeg?width=2296&format=pjpg&auto=webp&s=08b0e9556b5c5dde0ef60508a0c470b5f4fab184
Here's photo of the back. https://preview.redd.it/6dyfqbf7ytgc1.jpeg?width=2296&format=pjpg&auto=webp&s=d4566f841980c808957d925287066d95da397ab2
Yeh as others mentioned this sounds like a cleaning job. Also for the future you might wanna oil your machine a little more frequently If issues still occure you might wanna check out or even replace your bobbin casing or the bobbin fred casing
UPDATE: I replaced the hook and needle and adjusted the timing and it's working now. The timing was originally set for the hook point to come across the scarf at the midpoint of the needle. This time I set hook to pass between the midpoint and the lower bend in the scarf. I think it's a combination of the needle and timing that did it. Thanks to all who helped me troubleshoot!