What should I do if the upper thread tension is too loose?
If the upper tension is too loose, loops will appear on the back of the fabric. Increase the tension by turning the upper tension-control dial to a higher number. If the upper thread was incorrectly threaded, the thread may appear too loose, so rethread the upper thread following the upper threading instructions. (Page 1)
From the official Brother XH2177-001 Computerized Sewing Machine manual, page 1
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