SINGER W311 - Manual (Page 11)

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ADJUSTING THE TENSIONS
Always adjust the upper tension with the presser foot down,
as the tension is released when it is raised. To increase the
tension on the upper thread, turn dial (Fig. 13) to the right.
To decrease, turn to the left. Before adjusting lower tension
be sure that the machine is threaded properly.
When necessary to change the bobbin tension, turn the small
screw (Fig. 15) on side of the bobbin case clockwise to tighten,
counterclockwise to loosen.
Fig. 15
Fig. 16~A Fig. 16^B Fig. 16-C
When the upper tensions are properly balanced,
a perfect stitch will be formed with both threads
interlocking in fabric (Fig. 16-A).
When the upper tension is too tight, the lower
thread is pulled up over the upper thread which is
lying flat on the fabric (Fig. 16-B).
When the upper tension is too loose, the upper
thread forms loops over the lower thread lying flat
on the fabric (Fig. 16-C).
Fig. 17