How do I check if my bobbin tension is correct?

Remove the bobbin case and bobbin, then hold it by suspending by the thread and jerk it once or twice. If the tension is correct, the thread will unwind about an inch or two. If it doesn't unwind, the tension is too tight; if it unwinds too freely, the tension is too loose. Adjust the screw on the side of the bobbin case as needed. (Page 1 — Maintenance)