I had the same problem with a Saito 914S. The tank was at the centerline and everyone, that checked my plane, said it looked ok. I found a link that said the most likely problem was the engine was getting flooded as soon as I started filled the tank. So, I placed a clamp on the fuel line to the carb during fueling. Then I take it off just before I start it. The engine starts every time now. Just my thoughts.