Kenmore 1250 - Manual (Page 13)

Section I
Page 12
To check the thread tension balance, be sure to use mercerized thread of the same
size on the top and on the bobbin, and a sharp, correctly sized needle for the fabric
you are sewing. Begin with a full bobbin. Set the stitch length for about 12 stitches
per inch.
Fold a 6" square of fabric in half diagonally, forming a triangle. Make a line of
stitching W ' in from the fold.
a. If seam is puckered-both tensions are too tight.
b. If the bottom thread lays on the fabric-tighten upper thread tension, and
repeat test. See Fig. 3, Page 10.
c. If the top thread lays on the fabric-loosen upper thread tension, and repeat
test. See Fig. 2, Page 10.
Grasp the stitching and pull with a snap to make the threads break.
a. If both threads break-tensions are balanced.
b. If neither thread breaks-both tensions are too loose.
c. If upper thread breaks-loosen top tension,
d. If lower thread breaks-tighten top tension.
Both threads do not have to break at the same place, but they should break on the
same snap.
It is necessary to reduce the upper thread tension somewhat when zigzag stitching.
The wider the stitch, the less tension that is needed. This Is particularly important
when making designs. It is not desirable to have any of bottom thread visible on the
right side of the fabric when satin stitching.