SINGER W159 - Manual (Page 14)

When the upper and under tentions are properly balanced, a perfect
stitch will be formed with both threads interlocking in fabric. (Fig. I's)’
ALWAYS BE SURE THAT YOUR STITCHES LOOK LIKE FIG. 15.
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)
When the upper tension is too loose the upper thread forms loops
over the lower thread lying flat on the fabric. (Fig. 17)
ADJUSTING PRESSURE AND FEEDING OF FABRIC
GENERAL SEWING
Usually for straight sewing and zigzag stitching, the pressure bar cap or darner release. Fig. 18, is at its lowest
position and the drop feed knob. Fig. 19. is turned to “2” position.
Fig. 16
Fig. 17
Fig. 19