How to adjust tension
There are two types of tension adjustments, a basic adjustment for everyday sewing (this is what your repair person does when adjusting tension, but you can do it, too) and a temporary adjustment, necessary when you change thread types or sizes, fabrics, and stitching operations.

To make a basic adjustment, select contrasting colors of a thread in the brand, size, and fiber you use most frequently. Use one color to fill the bobbin, with the machine set on medium speed to reduce the risk of stretching the thread. Insert a new needle in the size you use most frequently and thread the machine, using all the thread guides on the machine head, but skip threading the eye on the bobbin-case finger if you have that feature.

Set the stitch length for 2 mm (12 sts/in.) or for the length you expect to use most frequently. Set the upper-tension regulator at the middle of its range (on most machines, this is 4 or 5), and stitch a test seam on two layers of lightweight muslin, then examine the stitches. If necessary, use a magnifier to see the stitches clearly. If the tension isn’t perfect, fix it by adjusting the bobbin spring; tighter if the bobbin thread shows on the upper layer, and looser if the needle thread shows on the underlayer. Make another test seam, and examine the stitches, repeating until the stitch is balanced.

Once your stitching is balanced, start a tension log in your sewing-machine manual, indicating the thread brand, size, and type, as well as the number on the upper-tension regulator that produced a balanced stitch. Then draw a picture showing the position of the bobbin screw, like the example below, to use as a reference if you need to record a change in bobbin settings for special threads.

	To record the bobbin tension for future reference, make note of the bobbin-screw position, including reference to thread opening or open side of bobbin case, as shown.

To make a temporary tension adjustment, select the threads for the needle and bobbin, then fill the bobbin and thread the machine. Make a test seam on the fabric that you plan to sew, examine the stitches, then see if you can find a balance using the upper-tension assembly alone.