What should I do if the upper thread tension is too tight or too loose?

If the upper tension is too tight, loops will appear on the surface of the fabric — reduce the tension by turning the upper tension control dial to a lower number. If the upper tension is too loose, loops will appear on the back of the fabric — increase the tension by turning the dial to a higher number. If rethreading does not solve the issue, also check that both the bobbin thread and upper thread were threaded correctly. (Page 1)