Diaphram carbs are SUPPOSED to operate in any position, however, in my experience it depends on the pop-off pressure and whether the engine is over/under carbureted. If the engine is overcarbureted (too large a carb) the pop-off pressure will necessarily be low to allow the low vacuum signal at the venturi to draw sufficient fuel. While the engine will operate fine in one orientation, the mixture will change in another. Why? Gravity will cause a small force on the diaphram in all positions. If the needle-valve spring pressure (which determines pop-off pressure) is low then gravity will alter the vacuum signal required for a particular fuel feed rate. Although this will happen with ANY pop-off pressure engine/carb combination, using a smaller carb with a higher pop-off pressure will minimize the problem.