I have read for many years that using a 1.12 size 4100-A Ford carburetor on a small displacement V8 was not satisfactory for most who tried. The usual commentary wonders why factory HP289 engines did so well with such a large carburetor. It is only recently that a few of us have started documenting what we find in vacuum secondary control systems, yes control, in various assemblies. Failing to find any significantly detailed Ford engineering information, even in Bob Mannel’s library, we are recording what we find. No surprise really but many carburetors that have ever been apart before are not just as they would have been manufactured anymore.
Stepping back a little, I have known since the 1970s that Holley® carburetors with vacuum secondaries used a control scheme to tailor secondary how much air flow through the primaries was required to trigger cracking the secondary throttle open, control how fast the secondaries opened, and control how much the secondaries would open in a given application. For a given engine size, Holley could have one “size” carburetor assembly crack open secondaries around 1,400 rpm and be fully open at 5,000 rpm and do another the same “size” that would not start cracking until around 2,400 rpm and never get fully open. That is the “shelf sizing” of vacuum secondaries you will read about, not really by itself until built that way. What Holley meant was something like an out of the box R-1850-1 will open differently between something like a 260 engine and a 430 engine.
As it turns out Ford 4100 and 4100-A models were created with the same types of devices to control how the secondary throttles worked. Holley published their parts and methods while Ford just skimmed past them in training manuals and the Master Parts Catalogs. Holley published how what parts do what while Ford hinted in obscure publications most vehicle owners would never seen. As it turns out Ford carburetors are adjustable too. A 1.12 size unit factory calibrated for engine X might flop the secondaries open super early on a 289 engine, while one calibrated for engine Y might never start opening the secondary. A sudden snap open at mid rpm and low road speed would make horrible driving manners for a 289 engine. Never opening negates having a 4V unit.
Our calibrated parts study is young. We are gathering up shared information, what Mr. Mannel had, and what I have found in Ford dealer and service information Ford used these features alone or in combination to control when, how fast, and how far secondaries operated. In the vacuum system circuit:
One, or two in later years for some large engine models, vacuum pick up source tubes. Models with a tube rely only on air flow through the primary side to create a vacuum signal to open the secondary. Models with two tubes delay secondary start up and once the secondary starts flowing both tubes supply signal to finish opening the secondaries at a faster rate near the end of travel. (Holley did the same thing in 1965-67 MUSTANG GT350 carburetors. R-3259, R-3259A, and R-3259AAS assemblies 1964-1965 had a single vacuum pick up while R-3259-1A and R-3259-1AAS models 1965-67 used two pick up points split between primary and secondary.)
Size of a calibrated orifice machined into the passage or size of the calibrated brass orifice pressed into the passage.
Include a 1/8” check ball or not.
Include a bronze coil compression spring with the check ball or not.
Depending on calibrated orifice diameter and use with or without a check ball the secondary opening characteristics will be vastly different. You would not want the secondaries of a 1.12 size carburetor to open quickly and or all the way on a small V8 unless hard acceleration is what you wanted to do.
I recently prepared a 1.12 size C4OF-AL HP289 carburetor for our black car. It was missing the check ball. I have a method of bench testing vacuum secondary carburetors before installation. For a 289/302 application I want the secondaries to open smoothly at a constant smooth rate in about three seconds of testing. Without the check ball the secondaries snapped open instantly. With a new ball installed opening was just under three seconds. That should be fine as the Holley model I took off had been tuned to open in right at three seconds. (The R-3259-1AAS was installed for a prior owner by his local Shelby dealer in the 1960s. The dealer ordered and installed a COBRA 4V induction kit. In the 1980s I tuned it to give world class performance under a wide range of conditions and fuels, street and track, around 600 to 700 feet above sea level. That development program is where I came up with my three seconds number from. I have tuned quite a few R-3259 family carburetors for GT350s and Cobras over the years and that three seconds number has been good for them all. I have done a few Ford C3OF-AJs for HP289s also. I will be testing the C4OF-AL this summer as the Holley seems a tad sluggish at low rpm were we live now at 5,400 feet above sea level. The smaller bores Ford carburetor should have more air flow speed through it at any given rpm and provide a little crisper tip in around town. We will find out. I have the Holley overhauled and ready to reinstall at some point in time.)
We are using a Shelby American 1964 race shop 4-2V induction system in our red car. Moving from 600-700 feet above sea level to 5,400 feet above sea level required resizing the carburetors, which is easy with 48 IDAs. As we moved there was not enough air mass to ever get the main systems started under 5,000 plus engine rpm. We had to change air bleeds and fuel jets also but with smaller chokes the main systems work properly at 20 something mph in fourth gear.
In choosing a carburetor "size" keep in mind how altitude affects naturally aspirated (carburetor(s) based induction systems) engines.
Dan
