I followed instructions tuning up the engine, including the line pinch test, all seems well, engine idles well, and accelerates without stalling, I have it slightly rich, A3 glow plug, 15% nitro w/ 18% oil, engine has had about 10 fuel tanks through it, good compression, I just installed it on a newly built trainer.

The PROBLEM I HAVE IS WHEN I HOLD THE PLANE UPWARDS IN A VERTICAL POSITION, THE ENGINE STALLS. When I put it down to horizontal it works ok this happens in all cases, idle, and full throtle.

I do see some bubbles in the fuel line, I will remove the tank to inspect for any kinks / holes in the fuel line, can there be another reason for these kind of stall?
Does the lenght / rise of the fuel line over the carburator plays a difference? The tank is installed about 1/2" below the bottom of the carburator, it's the only way.

Thanks in advance for your inputs.