0 should be pretty loose, check by pulling the thread backwards when fully threaded. When you have the top tesion between 4 and 5 it should stitch fine on two layers of quilt weight cotton. Do the drop test on the bobbin case just to have a rought idea of the situation. If you have switched to a heavier thread than your machine is adjusted for you need to do the bobbin tension over again. Make sure the top tensioner turns about 0-9 with out hickups, and that there's gradually increasing tension from 0-9. If not you are forced to take it appart and check it. Needle size makes the difference when stitching throught layers of cotton and batting.