Sailrite thread tension loops fabric - Sailrite Ultrafeed LSZ-1 Mechanical Sewing Machine

it is called bird nesting and is the result of the needle thread tension being too low
recheck that the thread is between the tension plates and not behind them
or disregard the settings and set ti manually
as if you are going to thread the needle , pull the thread and there should be a reasonable amount of effort needed to pull the thread
If there is not resistance to the pull then the tensioner is not applying the necessary tension

Check your sample. Is it the top thread that is looping under the fabric? That is called bird nesting or thread nesting. It happens frequently if the top thread is not seated completely in the tension disk. It could also be a top tension problem.

Be sure to use a brand new needle compatible with the thread and fabric.
Remove the top thread from the machine.
ALWAYS RAISE the presser foot and rethread from the beginning.
Set the top tension to the midway point and test on a scrap of the same fabric. Adjust the top tension until the top and bobbin threads meet in the middle of the fabric. (Tightening the top tension will pull the bobbin thread up. Loosening the top tension will allow the bobbin thread to pull the top thread down.)

Most likely there are jamming threads at bobbin room under the machine stepping pedal, try to dismantle it to get rid of these threads.

Between 3 and 4

If the thread is looping under the fabric, it is a top tension issue.

Make sure you are using a new needle and that the needle/thread/fabric are compatible.

Verify it is threaded correctly and that the thread is firmly seated in the tension disks.

If the top thread is still looping under the fabric, then the upper tension is too loose.

...

Is it actually the bobbin thread that loops or is the loop appearing on the underside of the fabric as you stitch? If it is a loop under the fabric, that's usually an indication it is an upper thread issue. Thread irregularities that appear on the topside of the fabric are generally bobbin related.

First thing to try is installing a brand new needle. Verify it is the correct needle for the fabric and thread being used.
All About Needles
Sometimes, the fabric being stitched will "grab" the thread and hold it for a second before releasing and will cause a loop. In this case, a needle with a larger scarf and eye may be helpful.

You might also open the upper tension as much as possible, RAISE the presser foot and run a piece of UNwaxed dental floss through the tension disk a few times to clear out any gunk that may be stuck there. A piece of pearl cotton dipped in rubbing alcohol and squeezed moist/dry will also work.

Remove the upper thread. RAISE the presser foot and rethread the top thread from the beginning. Use only fresh, good quality thread--NO old or bargain bin variety! Try using the same thread in the top and bobbin. Make sure the thread is firmly seated in the tension disk and the thread path is correct. Check that there are no burrs or snags in the thread path that could be catching the upper thread. Especially, check that the thread is not catching on the spool "slit" that holds the thread when in storage. Every time the thread navigates around the thread spool, it could be catching on that slit just enough to cause a loop.

Try changing the bobbin. There could be something amiss with the current bobbin--could be nicked or could even be the wrong bobbin for the machine. Also, make sure the bobbin is loaded and turning the correct direction in the machine.

Set the upper tension to the midway point. This is usually the starting point, although it will probably require some tweaking. Test your stitch and adjust the tension so the upper and bobbin threads meet in the middle of the fabric. Neither thread should dominate one side of the fabric or the other.

Understanding Thread Tension Threads

Last thing to try is a different presser foot.

If you still have the intermittent loop, you might want to take it for service. Could be a problem within the tension disk.

increase the thread tension of the needle thread
this is an indication that the tension is not enough to pull the bobbin thread up tight

try easing both bobbin and top thread tensions

looping the material ( bird nesting) is an indication of incorrect needle thread tension before threading the needle pull the thread and there should be resistance to the pulling

Put the tension settings back to the mfg's suggested setting.

Re-thread the machine with the presser foot up and there will be no more loops.