One more possibility. I do remember reading a post here one time about a crack in the remote needle assembly that resulted in air bubbles in the line. These would only show up in the line between the needle and carb. This might happen when vertical but not horizontal due to the extra suction required to lift the fuel. One more needle valve possibility is air leaking past the needle itself. I've had to add a short length of fuel line on some other types of engines to reduce this possibility.