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Zenith “PHYSICAL size’
letter meaning in carburetor type nomenclature
This article applies to downdraft Zenith carburetors and MAY or may not apply to updraft carburetors newer than 1930.
Approximately 1930, Zenith changed their style of assigning carburetor type or model nomenclature. This article is intended to help the reader understand the physical size portion of the nomenclature. The letter breakdown may be found in this link:
http://www.thecarburetorshop.com/Zenithcodebreakdown.htm
This breakdown applies to models with the following format:
P TTT MMMM SS R
P is a modifier that is often omitted as it was rerouted to be a part of MMMM
TTT is the type of carburetor
MMMM is the code which is explained in the link above
SS is the physical size
R is a modifier that is generally omitted, and when present, often refers to an internal governor.
Example B28V10R
B denotes a carburetor where the throttle shaft and choke shaft are at right angles
28 is the type of carburetor
V means the carburetor has a vacuum operated power jet (see link)
10 is the carburetor physical size
R means this carburetor has an internal regulator or governor
Example 28BV10R
Exactly the same meaning with the repositioned “P” modifier.
The following is (with permission) from a factory Zenith breakdown of the subject of physical size (the SS):
5 – throttle bore 13/16 inch, center to center 1 13/16 inch
6 – throttle bore 1 1/16 inch, center to center 2 ¼ inch
7 – throttle bore 1 1/16 inch, center to center 2 ¼ inch
8 – throttle bore 1 3/16 inch, center to center 2 3/8 inch (S.A.E. size 1)
9 – throttle bore 1 5/16 inch, center to center 2 3/8 inch
10 – throttle bore 1 7/16 inch, center to center 2 11/16 inch (S.A.E. size 2)
11 – throttle bore 1 9/16 inch, center to center 2 11/16 inch
12 – throttle bore 1 11/16 inch, center to center 2 15/16 inch (S.A.E. size 3)
13 – throttle bore 1 13/16 inch, center to center 3 5/16 inch
14 – throttle bore 1 15/16 inch, center to center 3 5/16 inch (S.A.E. size 4)
16 – throttle bore 2 3/16 inch, center to center 3 9/16 inch (S.A.E. size 5)
Remember that this chart is PHYSICAL size only. The airflow capability of the carburetor, while influenced by the physical size, is controlled by the internal venturi size. The internal venturi size is not referenced by the model designation, and should be acquired by referencing the identification (TAG) number of the carburetor.