margecam52

All of the suggestions so far are valid ones to keep in mind. Also remember to clean the bobbin hook area, bobbin case and anywhere else lint/stray thread may be hiding in. If this is the back of the quilt... then do make sure to put the hopping foot down...believe it or not...99.9% of all quilter's have left that foot up at one time or another. Most will tell you that you need to tighten the upper tension (hence check that the foot is down)...the usual rule of thumb is if you get loopies like this on the underside...it's the upper tension to loose...if they are on the top...the bobbin is too loose....but that is not always the rule.
I check my tension manually. I like to run all my threads fairly loose...for the top tension...I thread the machine, put the hopping foot down...and pull on the thread coming off the needle...the needle may hint at bending/flexing, then should straighten out and the thread should pull out straight and steady, no jerking and with some tension but no so much the thread breaks or the needle stays flexed.
Once I have that (and every thread is different, even color can make a difference) upper tension adjusted (righty tighty, lefty loosey)...I test the bobbin.
To check the bobbin manually, I lay the bobbin case (loaded with a full bobbin) upright (you can see the bobbin in the case) in the palm of one hand, and pull on the thread coming out of the case. The case will start to come off your hand, but will then stop and the thread will pull out straight and smooth...with just a hint of tension.

This bobbin test works the same as the one where you hold the loaded bobbin case above one hand and holding the thread, let the bobbin case go...it should slowly drop...like a spider on a web down to your hand.

I then test the stitches...I do this by stitching on a small scrap on the side of the loaded quilt. Couple of stars and circles...and the tension should look great..if loopies on the underside...you can tighten the upper half to one full turn and retest...if pokies...make sure they are pokies...run your fingernail over them...if you can't feel the pt pt pt of the pokies...they aren't pokies. If you can feel them...either loosen the bobbin 1/12 of a turn on the screw...or tighten the upper a half turn. If the upper thread breaks, back off a full turn and adjust the bobbin screw 1/12 of a turn (lefty to loosen).
Once you have a great stitch...you are good for that quilt/thread...you WILL have to make adjustments depending on the quilt fabric, batting, thread...it's normal to do so.
If you get a great stitch going & have the towa gauge for the bobbin...test the bobbin and jot that number down...then after that, no matter what size/type thread you put in that bobbin...you know the number you want that bobbin to test out at.