I'm not sure what you mean by making a knot. I had a problem with my 401 that would get tangled up and lock up the machine. I must have lost the photos when my computer crashed. What seemed to happen is that the top thread would (showing on the bottom) get longer and longer until it finally got stuck in the bobbin area and locked up the machine. You had to cut out the "knot" to free the fabric. By cutting it out, you couldn't determine what it was catching on.
This was my solution:
Cari, you were right!. It didn't totally solve the problem, but helped considerably. I would guess that it reduced the instances by 95%. Here's the youtube video:
https://www.youtube.com/watch?v=j5Y3UTg2Yj8 .
(Note here that my problem looked like increasingly longer loops on the bottom (which you can't see while it's happening.) until it locked up the machine by getting wrapped around something. This happened in the middle of a seam, not at the beginning. It may or may not be relevant to your problem. However, it's not difficult or expensive to try.)
bkay