??????????
Is it possible that you have confused a carburetor description with an air cleaner? If your 413 has a single inlet on the air cleaner, that isn't considered a ONE barrel. Some call it a single snorkel. If this is the case, there is help out there! Measure the diameter of the point at which the steel air cleaner assembly contacts the carburetor. Take that number and go to an auto parts store and ask for an aftermarket open element assembly. If need be, bring the original air cleaner assembly with you.