My Mother wasn't a sewer. There is a lot of fear about changing tension because it seemed to be a mystery as to how to get it back on track. Mechanical machines must have the presser foot down when you change the top tension. Don't ask me why, I really don't know, I just know it works. If you move the tension on top with the presser foot up, you basically have changed the numbers of your tension. You can change the tension to make it tighter or looser. The bottom tension, has 1 little screw that can become stripped. So if you want to change your tension with the bobbin screw, be sure you have access to another bobbin case. The recommendation for bobbin cases is have 1 you change the tension on, and mark it. Generally a little mark with some fingernail polish where the screw slit starts and ends. That way you can change it back to factory setting. I have found that in addition to your tension knob your tension may be having problems because of the thread--bottom weight thread and thicker thread in the top, needle problems, quilting for a while always seem to make the tension go wonky too.