If the knot is on top there's too much top tension on my machine.
I'll pull the top and bottom threads by hand and I have a little more bottom (bobbin)tension than top tension but they're pretty close to being the same.
I leave the bobbin tension alone and just adjust the top tension when I have a problem like you are experiencing.
Knot on the bottom, tighten the top tension, knot on the top, decrease top tension.