Try switching the lines at the carburetor and see if that makes a difference. The longer line in the fuel tank needs to be long enough for the filter to move around (model airplane people call them "clunks") and pull fuel from the bottom of the tank no matter the orientation of the engine. The shorter line is the return line from the carburetor and allows the primer bulb to "push" fuel back into the tank. I had one case where the primer bulb didn't return to normal and it was due to a plugged screen in the carburetor but don't go there yet, just try switching the lines at the carburetor and report back what you find.