I had alot of trouble getting the tension on my Tin Lizzie just right and this pull test helped alot. Now my stitches are beautiful!
Test the upper to start...put the hopping foot down, have the machine threaded...pull the top thread out from the needle...The thread should be fairly hard to pull, but should not break. If it breaks, turn the tension dial a full turn to the left (lefty loosey, righty tighty). If the thread is fairly easy to pull out...tighten it a full turn, and do the pull test. Once it's so tight that it breaks, loosen it a full turn. The numbers on the tension dial are NOT settings, they are referrence points...so if you start with a 2 at the top, a full turn will again have the 2 at the top.
Once the tension seems ok...pulls out with resistance, but not breaking the thread... work the bobbin.
Lay the bobbin case (bobbin showing/facing up) in the palm of your hand...pull the thread coming off the bobbin and the bobbin should hint at coming off your hand (will start to move)...but then the thread should pull out with just a slight resistance.. .with the bobbin staying in your hand.
Now test the stitches...if you get loopies on the bottom, tighten the upper 1/2 turn...and try that...if the thread breaks...back it back a half turn ...and loosen the bobbin screw 1/16 to 1/12 of a turn. That should be about the right setting. Test the stitches.
If you still have bad stitches..check the bobbin case for thread/lint. ..make sure to use the pigtail guide on the bobbin case.
Try flipping the bobbin over in the case (it's not real important which direction the bobbin spins...choose what direction works for you). Also try a different bobbin...yours may be slightly out of round.
I've attached a word doc for you to print if you need this info.
Tension problems word doc