Margaret,
When I acquire a machine I usually, after an initial test run, start with the bobbin tension by turning the tension to max tight and then turn the screw CCW about 1/2 turn. At this point I will pull, once the bobbin is installed, on the thread and by feeling the amount of tension required to extract the thread from the bobbin I would stop any additional adjustment. Begin threading as if ready to sew. When the thread is through the needle eye, begin pulling on the thread. At "0" setting there should be little or no tension. Now continue increasing upper thread tension a little at a time and if all goes well, you should get the right amount of tension around the mid-range on the dial, say around 5-7. Run a test after each adjustment and see if the stitch locks between the 2 layers of material. With 21 machines in my collection, and not scientific, this works for me. This is a trial & error process. Any adjustments must very small, especially the bobbin, maybe a 1/12 turn at a time. Hope this helps out.
Rodney
Ponchatoula, La.