Holley has always offered vacuum-secondary performance carburetors in two versions--4150 models with replaceable jets in the secondaries, and 4160 models with a metering plate and no replaceable jets. When the time comes to re-jet the carburetor, it might be a big deal. Simply put, a Chevy equipped with a 4150-series carb is miles ahead of one with a 4160. In the 4160 carburetor, changing jet size mandates replacement of the thin metering plate, as there are no conventional jets to replace. Because of that, a theoretically simple task becomes downright tedious.
Holley offers a simple conversion kit (PN 34-13) that converts 4160 carbs to 4150 models; it is designed for race-style (cathedral) bowls. For vacuum-secondary carbs with transfer tubes (e.g., 0-1850 carburetors), you'll need Holley conversion kit 34-6.
Metering-Plate to Jet-Number Conversion
The majority of metering plates are stamped with an ID number, but they do not exactly correspond to jet-orifice numbers. Also keep in mind that metering plate ID numbers may not be sequential. For example, a No. 21 metering plate is actually larger (in terms of relative jet size) than a No. 37 plate. There are other snags, too. In the following chart showing the relationship between metering-plate numbers and jet sizes, you'll see that two different plates carry the same ID number.