I think I figured out my own problem. The screw to the left of the arm, in the picture, goes through a thick metal spring under and inside the bobbin case, which puts pressure on the pivot point of the arm when up or down. There is supposed to be a very small nut on the underside of that screw, not there in this case. The bobbin bouncing all around in the case would explain, in part anyway, why I couldn't sew good stitches with this machine.