Hi, what's the difference between the upper and lower carburettors on the above engine? Is there a way of identifying which one is which when they're taken off the engine? I'm playing with a carb I took off, I am fairly sure it is part no. 435910, and has a number stamped on the body, 339169 D2. I took it from the upper position, but it looks the same as one I've seen for sale which is said to be a lower carb.