I ran my M22 out of fuel. I have an internal tank in it. So I filled it back up and started it. Once it went to high for a minute it shut down again. I could hear the fuel pump knocking so I assumed it needed to be primed. I primed it and started it. It then did the same thing and shut down. At this point I figured the fuel filter was clogged. So I cleaned the screen where the tank sits and removed the fuel filter. It was partially clogged. I cleaned that and restarted the heater. All is well now.

So if you run out of fuel and can't get it to restart, prime it, start it again, and then listen for the fuel pump to cycle after a couple minutes. If it keeps knocking every time it shuts down the fuel filter could be clogged or the source tank is not feeding. I just thought I would share my experience.