I had a similar problem many years ago,,, I forget the engine. The problem was in the tank. Choking the carb would bring fuel up the line OK. As well, the engine would run out the prime. But that's all. The problem was that the fuel line in the tank was collapsing when the suction of the engine's prime run pulled hard. That was WAY back in the 70s, as a matter of fact. We used surgical tubing then and eventually, it would go soft if you left the tank with any amount of left over fuel in it. Silicone was just coming in at the time.