Once I had the machine all back together, I tested the stitch. Omigosh. There was an awful racket from below. I was sure that I had not pushed the position bracket back into place. Nope. All was fine. The top tension felt terribly tight. I took it apart, AGAIN, honed the discs and reassembled it. (Which is why the check spring isn't quite right, it was perfect the first time). Still no better. I took the slide plate and the needle plate off and watched the thread as it made its journey. It was catching in two places. First just along the bobbin case and again in the back.
Work with me here, imagine that there is a bobbin in the machine and there is thread catching along that path. The answer to the problem is in this photo. The clue is in the title of this post.
I had tightened the bobbin case position bracket too tightly and there was NO ROOM for the thread to escape along its journey to making the stitch. In this photo there is some space space between the bobbin case and the position bracket. I took this photo yesterday before I started messing with the machine. This space is the escapement. There MUST be space there, or thread will catch.
(No, the whisper was not the noise of the position bracket rubbing on the bobbin case. Those two pieces are stationary)