Have you tried to blow through the lines with the tank empty? Maybe there's some kind of obstruction. If your needles are that far backed out and it's running properly then it's not getting enough fuel. There are a ton of reasons that could be happening.
If it always happens at 1/2 a tank of fuel there could be a cut in the feed line in the tank at about the half level where it start sucking air. It could be there's some dirt in your carburetor. I would put the engine on a stand and see how it works there and completely rebuild the tank with new tubing (metal and silicone).