The function of the idle system is nothing more than a "team" of items working together to provide sufficient idle fuel to the engine at low rpms at the given vacuum being produced by the engine.
I covered the "basics" in the book, and how to modify the idle system to compensate for engine modifications.
The idle down-channel restrictions in particular, are located before the mixture screw holes/lower idle airbleeds, and after the idle tubes/upper idle airbleeds. They do as mentioned, put a restriction in the system mid way between the idle tubes and mixture screws. The size of the idle down channel restrictions varied considerably on factory carburetors, as did idle tube sizes.
In very early years, idle down channel restrictions were very small, as were idle tube sizes. These carburetors also used very tiny upper idle airbleeds, yet still delivered plenty of idle fuel. Common sense would tell us that the smaller airbleeds mixed less air with this fuel, and at the same time allowed engine vacuum to pull harder on it at the same time. Since we saw larger idle airbleeds being used as emission regs continued to get tighter, this would indicated that more air available to the idle fuel as it made it's way to the mixture screws was effective at reducing emissions.
This fact also made later quadrajets difficult to use on engines with significant modifications, such as lareger than stock camshafts. Since aftermarket carburetors were not targeted for emissions, they were sought out by enthusiasts to replace factory units.
The good news here is, that we can go in and make changes to the q-jet to get the idle system up to par, and overall it will work as well as and more often better than any aftermarket carburetor, and save money as well....Cliff
