Pfaff 1151 bobbin tension

Question

I'm having a problem with the bobbin tension. The stitch looks fine on the front and on the back it's loopy. I've tried tightening the top tension which gets a bit better. I've loosened the bobbin tension and tightened the bobbin tension. The machine was working fine until I ran out of bobbin thread and put in a new bobbin. I've tried putting the bobbin in as it shows in the manual. I've had this machine for fifteen years and know it inside and out. This one is throwing me for a loop!