The above carbs have a rare 120-429s main jet, and two other features, one good, the other bad. The bad first. Hot idle compensator (Carter name Hot idle valve), a thermally controlled vacuum leak to under the throttle valve. In theory would begin to bleed air in after a certain temperature. Higher temp, more air, etc. Did not work right all the time and only lasted 2 years. The air passage should be blocked in the main body to prevent problems.

The good: around 1968 [close guess], two things changed; emissions rules and different cam, just a little more overlap. Now this is my personal theory. A stock idle circuit in a stock carb is made to run with a stock cam. Anyway. Carter did really big changes to idle circuit starting around 1968. First off, idle slot much longer, goes all the way to gasket line. Second, larger idle orifice, .035 and .039. Next, idle air bypass. This is an internal passage inside the carb body, not through the venturi. Gets air for idle while keeping throttle more closed and out of idle slot. In theory the idle mixture screw should work better this way? Last, the carb has a spring loaded against vacuum adjustable idle air bleed. Low load, high vacuum opens the bleed. More air leaner, low vacuum high load bleed closes, go rich.

There is a screw under a plug at top of carb, partly under the air cleaner gasket. Removed plug will not cause vacuum leak. Count number of turns or half turns. 1 turn clockwise should show results. The ideal mixture will need to be adjusted.

Well that did not work, I'll try to find pictures later.

The first picture shows hot idle valve just to left of choke vacuum pull-off. It is just under that cover plate, held on by 2 screws.The third picture shows idle air plug at top next to hole for air cleaner support.

Factory cams were as follows:

225 from '60-'64 and 170 from '60-'65 had a cam with specs 232° intake duration, 228° exhaust duration, 0.37x" lift, and 8° overlap.

225-1bbl from '64-'70 and 170 from '66-'69 (+'70 export) and 198 in '70 had a cam with specs 241° intake duration, 236° exhaust duration, 0.39x" lift, and 16° overlap.

225-2bbl (export only) from '67 on and 225 (domestic) from '71-'80 and 198 from '71-'74 had a cam with specs 244° intake and exhaust duration, 0.420"/0.414" lift, and 26° overlap.
Yes.
No, this was done to give better fuel atomization at idle for more consistent, more complete combustion with resultant cleaner exhaust. Not just Carter; Rochester and other carb companies did the same. This cross-section shows Rochester Quadrajet idle systems, '68 (conventional) vs '69 (bypass). The white arrows represent air; they're hard to see against their light-grey background, but study the image and you'll see that air through the bypass hits the fuel much more forcefully than air around the cracked-open throttle plate:



By the way, Matv91, please check your PM box; you've got a message waiting there.

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Too many people who were born on third base actually believe they've hit a triple.

Under that plug is screw for adjustable idle air ,the circle under that is the air supply for the adjuster.