Sewing machine bobbins hold the lower thread that loops with the upper thread spool through the sewing machine needle, creating a stitch that secures, typically, two layers of fabric together. Sewing machines provide mechanisms for tightening or loosening both upper and lower threads. However, while the upper thread tension adjustments are clearly marked and easily accessible on a Husqvarna sewing machine, adjusting bobbin thread tension can be tricky. Ideally stitches will look the same on both sides of the fabric. But a too loose or tangling thread stitch requires immediate attention.
Determine if the stitch problem is in the upper or lower threads. Generally, if the thread is bunching up under the fabric, the upper tension regulator dial should be adjusted.
Check that the spool threading of the upper machine levers and guides is correct.
Check what type of needle is in the machine. If sewing on sheer or knit fabrics, and missing stitches is the problem, the universal needle should be changed out for a ballpoint needle. However, if problems persist, it's time to work on bobbin tension.
Tighten the bobbin spring screw with a small flathead screwdriver (provided with the Husqvarna tool kit) a fraction to the right---or clockwise. The bobbin spring screw is located on the perimeter of the bobbin case.
Test sew a fabric section.
Continue tightening the bobbin case screw in marginal increments until the stitch is the same on the top and bottom of the fabric.
Loosen the bobbin case screw with the flathead screwdriver by turning the screw to the left---or counterclockwise. Again, do so in small increments.
Test the stitch on a piece of fabric after each adjustment. Keep in mind that thicker threads will exert more pressure on the bobbin case, and will require loosening of the case screw.
Continue loosening the bobbin case screw in marginal increments until the stitch is the same on the top and bottom of the fabric.
Adjusting the tension on the bobbin case is a matter of trial and error. Be patient, and turn the screw in small degrees, testing the stitch on fabric after every small turn. Some sewers like to keep two bobbin cases handy; one for normal weight threads, and one with a slightly loosened case screw for heavier threads.
If there is no screwdriver with the sewing machine, take the bobbin case to a hardware store and find a flathead screwdriver that will fit in the bobbin spring screw.