How do you start and stop when you FMQ, without having a rats nest on the back side? I have tried tieing off but when I cut the threads the knot comes undone.

When you begin, take one stitch and pull up your bobin thread. Hold it while you begin to stitch. When I finish I either take a back stitch or I leave a thread long enough to pull through and finish it with a stitch and hide the thread in the batting. If your machine is leaving that much thread on the back your tension might be off. Sandwich some fabric and batting and practice a bit before you start on the quilt. I do this every time when I begin and after I've changed bobin just to be sure I don't have a problem on the quilt. I always use the same thread in the bobbin (or at least the same brand/weight) as I use on top. Hope this will help. I know it's terribly frustrating.

I agree with this. These suggestions have helped me avoid that problem. And I FMQ on a dinky mechanical Brother with coats and clark thread.

Nana Jan said exactly the correct things.

If I'm unable to hold the thread I take a pin and stick it in the quilt, wrapping the threads securely around it. It holds when I can't. Take a couple securing stitches before you take off.

sounds like you are not pullling the bobbin thread up and holding it. I have started leaving about 6" and burying the ends in the quilt. Takes a little time but works.
If I am free motioning I will hold the fabric and let the needle stay in place for a stitch or two. If you are straight stitching with the walking foot, then set the length very small for the first three or four stitches and then dial back to the size you want.
If free motioning, pull your bobbin thread up and hold and just rock the needle back and forth to make really small stitches.

I start quilting off the top in the batting and them move onto the top. I do the same thing to stop quilting. I quilt right off the top into the batting. IF I have to stop in the middle of the quilt to change a bobbin. Then I take A stitch and then do as the others have said to pull the bobbin thread to the top. Once I am done with the quilting I will then tie a knot in the threads and thread them through a needle and then hide them inside the quilt. kinda between the top/in the batting. They don't come out and you can not tell where you stopped and started!

What I do is turn the wheel and needle by hand and slowly and that actually helps get rid of the nests.

you need to bring your bobbin thread to the top and hold both threads when you start sewing. then you go back and thread those (2) threads onto a needle and (weave) them into the center of the quilt

also make sure the presser foot is in the down position before you start sewing. that also would make rats nests if it was left up.

Thanks for all the help. I sure will try it and hopefully not have to pull the quilting out again.