After I aquired a check valve and tried it on the pressure line, that didn't fix it. I then tried it on the fuel line, and that stopped fuel from being sucked back into the tank. But it didn't fix the running problem. I removed the fuel tank, and hooked it up outside the airframe, so I could move it up and down, trying various heights. Guess what, still not fixed. I called Horizon, and they suggested a hotter plug after I explained all of the different steps I have tried. I never thought of that one. So, it looks like another few days before I get to try that theory out. The motor runs fine upright, along with the first one I tried. It seems to be an issue of running the motor with the cylinder horizontal. Any constructive comments are welcome, I am almost out of ideas.....