Zenith built a series of cast-iron updraft carburetors in the 450-550 series, including models 455, 456, 457, 554, 555, 556, 557 and 558. The models differ in bore size, gasket and needle/seat design, as well as calibration. There are three different floats used.
The 450-550 series was used from about 1934 to 1955 and was on trucks and equipment, including military, as well as marine engines such as Chris-Craft, Chrysler, Gray Marine and others.
Marine models are designated with the letter 'M' and can be distinguished by an upturned air inlet.
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How to find Zenith carburetor numbers