Model 4075 white sewing machine thread from bobbin builds up and stops sewing.

Check the stitching and confirm that it is in fact bobbin thread and not the top thread that is creating knots & loops under the fabric. This is usually the case and is called thread nests or bird nests. (You can search the internet for more information about this symptom.)

Install a brand new needle.
Remove the thread from the machine.
ALWAYS RAISE the presser foot while rethreading the upper thread.
Confirm the thread path is correct.
Verify the bobbin is installed facing the correct direction in the bobbin case and the bobbin case is threaded properly.

Set the upper tension to the midway point (factory standard) although it may need to be tweaked as machines change with use and time.

Pull up the bobbin thread. This method works with general sewing projects, not just quilting.
How and Why to Bring up the Bobbin Thread

Begin to test sew slowly.

If the thread is still making a mess, then the machine most likely has a timing issue. There are multiple web sites and videos that discuss sewing machine timing. Or take it for service.

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