ive got a carter afb carb on my 63 lincoln 430. it has the 2 idle adj screws but what is the larger acrew between the two?
Carter used the adjustable idle air bypass for idle speed control on a number of different AFB carbs; for Cadillacs, Chryslers, and Pontiacs (maybe others) as well as your Lincoln. As the properties of modern fuel are different than real gasoline, sometimes the only way to adjust the idle is as Jeff mentioned. There is a "dimple" on the pump side of the carburetor that may be drilled and tapped for a number 10x32 machine screw to use as a curb idle screw. If doing this on a relatively stock engine, simply close the air screw. However, these things are very useful in the hotrod sense; as one has a a built-in adjustable idle bypass to aid in adjusting idle if the engine is modified with a "big" camshaft. Remember the holes drilled in the primary throttle plates on some performance carbs??? No need to drill holes, as the large screw can be used for the same thing, and it is adjustable. Jon.
If I don't learn another single thing all day I'll still be happy! But I think I remember seeing years ago, one of these carbs (or maybe it was an AVS?) that had just the single idle air adjustment screw front and center and no idle fuel mixture screws. A friend bought it very cheaply from some mailorder surplus place and it appeared to be brand new. I seem to recall that it still had the normal idle speed adjustment screw. Someone pretty knowledgeable about these things suggseted that it was for a marine application. Does any of this sound plausible or am I suffering from Faulty Memory Syndrome?
Some of the AVS carbs had a single screw at the top (left hand thread). Smog emission carburetor. Jon.
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