Can anyone explain the function/use/need of the screw circled at the base of the carb? It's a 5/16 SAE thread and has something to do with air control in the idle circuit. The 4GC carb I got is missing it.
From what I can find it's an " idle air adjustment" it appears there was a fixed idle air restriction and an adjustable , I'm guessing to be able to lean the idle emulsion before it gets to the idle adjustments ..... I googled " Rochester 4gc air vent adjuster"
If I remember correctly, Pontiac used the 4GCs with the air bleed, maybe also Cadillac. You might be able to narrow your search by those makes.
My early Cadillacs and Oldsmobiles do not have one. But I'm like hodrodart I remember AFB Pontiacs and a 1965 Cadillac Rochester 4GC having one. It was the year before the Q-jet and the Cad had a small set of front venturii's and a much larger rears but not quite as large as the Q's. Carb King will know and chime in here I'm sure
Rochester called it an "Idle Air Adjusting Screw". Was used by both Rochester and Carter on certain specific carburetors in the 1950's and 1960's by several different car makes. Rochester used it on both the 4G (4 barrel) and 2G (2 barrel) models. Carter used it on both AFB and AVS models, as well as early WCFB carbs. Was used to set idle RPM instead of using a throttle posititioner screw. To understand its use, think of it as a metered vacuum leak. What happens to the idle with a vacuum leak? The idle increases. So the theory was that by having a controlled vacuum leak, one could adjust idle RPM. Can be difficult to set idle with this once gasoline was diluted with ethanol as the BTU content of the ethanol is less than that for which the carburetor was designed. We used to offer a modification to our customers when we were still restoring carbs of simply adding a throttle positioner screw. With the throttle positioner screw in place, the idle air adjusting screw would simply be bottomed out with no leak. The screw had a really useful function for the hot rodder with a big cam on the street. Think of the factory RA Pontiac carbs with the idle air holes drilled in the throttle plates. Lots of carbs have been modified with throttle plate holes to adjust the throttle plate angle for use with big cams. Often, the beginning tuner keeps drilling the holes oversize until the holes are too large, and then has to replace the throttle plates, and start over. With the idle air adjusting screw, no holes are necessary, and the screw can do the adjustment. Originally used on some GM applications, but Chrysler used it in the 1960's on many of their Carter AFB applications. Jon.
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