Tacsew T500. I have good stitches then open stitches. I've tried tightening the tension and also decreasing the tension. Nothing seems to work. HELP!!

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Sounds like you may have burrs on the throat of the needle plate. These are caused by the needle striking the sides of the opening, either from pulling the fabric or the fabric not feeding freely and are frequently, though not always, associated with a broken needle. Any time you hear a "chunk" while sewing, your needle has hit the plate and probably left a burr. These burrs put drag on the fabric or on the thread and cause the issue you described.
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