clumps of thread

If the thread is clumped up under the fabric, then the problem lies somewhere along the upper thread path, which includes, but is not necessarily, the tension. The problem could also be the thread getting 'hung up' somewhere, burr on the needle, etc. The thread getting cut off that you mention is likely the source of the problem -- find out where and why this is happening. It may be happening in the tension discs themselves, i.e. if there is a burr or renegade piece of thread caught in there.

-Irene

-------------- You only live once, but if you do it right, once is enough.=20

--Mae West=20

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Change the needle & be sure it is appropriate for the fabric. (Change needles every 5-6 hours of active sewing) Rethread the machine with good thread. Check the bobbin area & under the stitch plate for loose threads & lint. If the machine is oilable, has it been oiled lately with a proper sewing machine oil? If you machine requires manual tension adjustment, check that. Birds nest on the bottom can mean too loose top tension. In addition to needle burrs, thread can also break from improper tension and either the bobbin or upper threads not being threaded properly.

Another thing I find you need to consider with these cheapie machines is the thread take-up lever: Have you threaded that? And if so, has the thread jumped out?