Can you see the fuel line going to the engine? Any bubbles would indicate a damaged line. It's pretty common to damage the clunk line at the stopper when assembling the fuel tank if you didn't put a smooth radius on the edge of the tubing. That would cause the problem you are describing. Less likely is a bad pump or diaphram (yes, it's possible but not likely with a new engine), some dirt causing the cover over the fuel pump not to seal properly (easy to check and fix) or the pop off pressure not set properly. I had an issue with the latter with my Zenoah. It would run on the ground just fine, would idle for days, ran with the nose up and nose down, but would dead stick after a couple minutes of flying. I was lucky enough to have a bona fide 2 stroke engine guru in the club, a really nerdy former semi-pro race driver who had started out with 2 stroke go carts and graduated to sports cars later. He even raced against Jamie McMurray at one point in his career when McMurray was 15 or so. He now owns a car restoration business that receives work from all over the world, so when he talks about engines I listen. He explained the importance of pop off pressure to me and showed me how to test it and set it properly. The engine runs like a top now, so it would be worth checking on yours too if the fuel line idea doesn't reveal anything.