Carter afb idle air circuit mod.

Anyone ever done this to an afb?

“The solution to idle quality on "driver" vehicles (not numbers matching) is to install a conventional idle control. There is a "dimple" in the lower casting on the front drivers side (where the normal curb idle screw would be located). Drill through the dimple using a number 21 drill, and tap for a number 10 x 32 machine screw. Obtain a 10 x 32 machine screw and spring at the local hardware or auto parts store. Install the screw and spring. Screw the idle air screw all the way in, and then adjust the idle the conventional way.

If you wish to leave the adjust as the factory suggested with the idle air screw: for MOST applications, the beginning settings would be 1 turn on the idle mixture screws and 1 1/2 turns on the idle air screw.

Where these carbs are really useful is for those using big cams (low vacuum). The idle air screw, used in conjunction with the conventional idle screw is an adjustable idle bypass. Remember the holes drilled by the factory in the throttle plate on some of the performance carbs? No need to drill, as the idle air screw will do exactly the same function, and it is adjustable.”

got any pics? the only "dimple" type passage I notice on the driver side is the plug for the accelerator pump passage.

I believe that is for '68 and some '69 AVS carbs. Those are the only ones that I know of that have the idle air bleed screw and blocked off idle mixture screws.

I got the description. For this mod off another web site so i done have pics.

I was ready to drill holes on my thorottle plates when I ran across this

Yeah, its the 68 AVS with the single air/fuel mixture screw.

This is referring to early AFB’s. My ‘66 Hemi has them. There is not an idle speed screw, only an air mixture screw on the front.

Stan

that large brass screw in the front of the early carbs was air volume while the two mixture screws were for fuel. this is the only way I can think of to get enough air at idle and still keep the throttle blades closed. 3447 s/s carbs had them, 273 hipo's , and i'm sure others. the pcv connection at the front of the carb took the place of the brass air screw and a curb idle screw was needed to open the throttle blades. there's probably a way to create a controlled leak at the back of an edelbrock carb. I know some class racers have used pcv valves to create leaks.

The easiest way to get a finer tune on your idle mixture on the 68-69 AVS carbs with the single upper idle screw (which adjusts in the opposite direction as the lower screws) is to disable the upper screws (remove air horn, add lead plugs to the left and right passage ways) which disables the upper screw, then open the lead plugs below that are in the conventional lower idle screw location, small recessed mixture screws will be revealed which can be adjusted and left as found, or they can be removed and replaced with two conventional screws and springs. I have done this to many of them that arrive with stripped upper screws (from turning them the wrong way etc.) and it works just fine.
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