The Zenith model 28ADA (28 duplex) carburetor were two barrel downdraft carburetors, built mostly in Zenith size 10 (SAE 1-1/4" nominal), with some Zenith size 8 (SAE 1" nominal) listed as well.
This series of carburetors was used on many different industrial and agricultural applications, and were equipped with manual or automatic chokes. They were also available in universal models and sold as factory replacements. Production was from the 1940s into the 1970s.
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How to find Zenith carburetor numbers