There are at least a couple of reasons for different models:
(1) different internal venturii sizes
(2) maybe slightly different linkage hook-ups.
From my post above: "This just explains PHYSICAL dimensions. For air flow, one must consider the internal venturi size which Zenith expresses in millimeters."
I don't know the exact range of venturii sizes available for the 228 size 12. I DO KNOW they offered a 28mm and a 40mm, with several inbetween.
Air flow in CFM (measured by Zenith):
28mm - 177
31mm - 214
34mm - 220
40mm - 265
Turn the clock back 70 years, and these were readily available from Zenith dealers (10 cent gasoline was also available
). I doubt very seriously if one will find much of a selection anywhere; but a good machinist can make them in different sizes.
The venturii in the 228 size 10 differ physically from the size 12. The chances anyone is going to reproduce these things in quantities great enough to get low prices is less than the probability of winning an argument with the IRS !!
So, if one wants to change venturii, one purchases various 228 size 12 (LOTS of applications other the GMC) for parts, or has one fabricated.
A common venturi found in the 228 size 12 GMC units is 34mm but they used both smaller and larger, depending on the mass of the vehicle, and the desired RPM of the engine.
The Carter 2100s YF many find desirable for the 235 Chevrolet, and some run on a 261 has a 1 5/16 inch venturi. Converting to metric is a tad over 33mm; thus the typical Zenith GMC 228 with a 34mm venturi will flow marginally more air.
The 228 does have a feature not found with the Carter YF. While virtually zero original equipment Zeniths came with an externally adjustable main jet; generally one may be added after the fact. If one lives in a mountainous terrain, and changes altitude frequently; one can recalibrate the carburetor for different altitudes simply by adjusting the screw. A second feature is a conventional accelerator pump, not a diaphragm.
Zenith produced "index cards", which are double sided 8 1/2 x 11 inches, ONE FOR EACH CARBURETOR. These things are virtually impossible to find.
Zenith also produced "Master Parts & Service" books FOR CARBURETORS IN PRODUCTION AT THE TIME OF PRINTING.
To get a list of most of the Zenith carburetors with their specifications requires the purchase of several different books printed at different years.
I have posted for decades that the Carter YF is an excellent, inexpensive carburetor (although those used to Rochester prices might get "sticker shock" when pricing a Carter
). The Zenith and Stromberg both are a better quality carburetor (OK, call the YF 92 out of a hundred, and the Stromberg/Zenith 96 out of a hundred) especially with ethanol diluted fuel from a reliability standpoint. But they are generally more expensive than the Carter.
And if I had a stovebolt 261, what would I use?
(3) three Carter type W-0 carbs on a normal 3x1 intake running solid linkage (sorry Jerry
) or a Corvette intake with three (3) Stromberg or Zenith sidedraft carburetors.
Jon
