I'm trying to list a pair of Weber dual carburetors for a buddy of mine that go to a old VW Bug/porche, the only part numbers I can find on them is 3012L and 3012R. Now I know the L and R is left and right but can't find out what the size of the carbs are. They made them in 34mm, 40mm, 44mm and 48mm, how do I measure them to tell size, cant find out anything in the computer any help would be greatly appreciated. thanks