I have had the Juki DX7 for a few years now and haven't had to adjust the bobbin thread. Try very small increments in the top tension. I use Aurafil 50 in both top and bobbin but I use Topstitich 14 or Microtex 14 needle. What foot are you using? I use the open toe free motion presser foot but I bought that as an option. Good luck!