Sewing Machine Thread Balance

Do a line of stitching with one color on the bobbin and a contrasting color on the needle from the spool. The two threads must look halfway between the layers of material. Try to find an equal tension using first the needle thread and then the bobbin tension. My home ec teacher would always say that the bobbin tension never needed adjusting. It would perhaps have some thread in the feed but the tension us usually not the problem. 

If the bobbin thread shows on top in little loops at the ends of the stitches, the tension on the needle thread is too tight and is pulling the thread up from the bobbin through the material layers. Decrease the tension. If the opposite is true, tighten the needle tension and that should draw the bobbin thread up into the fabric.

The bobbin case has a screw that will adjust the thread tension. With a small screwdriver, tighten thread tension by turning it to the right. Loosen by turning to the left. Always test the top, needle tension first as that may remedy the problem and check for any thread or fabric fuzz that might be stuck in the thread feed. Use your handy little long bristle brush to chase out any lingering fuzz balls.

The simple test of bobbin tension uses the bobbin in the bobbin case as you would usually prepare it. Hold the thread in one hand with your other hand under the bobbin case. Drop the case to your hand with the weight of the bobbin case and the tension on the spring in the case, it should hold the thread and case taught  without sliding freely. If it falls easily, the tension is too loose. If you have to pull to get it to move, the tension is too tight. Too loose, when it pulls easily.