AFB with holes primary throttle valves
 

A tale of two (supposedly identical) 3300S carbs for my '63:

One has a small (1/32"?) hole drilled in each primary valve at approximately center, but just aft of the screws. The other does not.

The one with the holes was the one I had on the car from about 1976 until 2004, and which I rebuilt myself once. I have no recollection of noticing those little holes (that would have been 40 years ago or so, and as I don't normally get to see scads of AFBs (or any other carbs) on a regular basis, I might not have even thought anything about them.

The carb without the holes is a fresh core. I do not know its history.

Can anyone shed some light on this?

TIA

Those little holes are often added to the primary throttle blades for several reasons and may be drilled by the factory on some models. They are often used for the following: 1. to allow air to bypass the throttle blades for more idle air while allowing the throttle blades to remain closed and not expose the transfer slots. 2. Used with larger cams that don't make enough vacuum at idle to keep idle speed up and steady. 3. To keep the throttle from "sticking" with the blades completely closed.
I don't work on enough AFB's to know if they used drilled throttle plates from the factory on certain models. From a logical standpoint, I wouldn't think this feature would be needed on AFB's with idle air bypass. (the giant center adjusting screw in the center on some AFB's). Nor sure if any of this info helps.
If you install the carb and it won't idle slow enough for your application, then non-drilled butterfly's may need to be installed.

Thanks!

This is all quite logical and pretty much confirms my suspicions, based on my thinking of what would result from a hole in the throttle plate.

I guess we'll try one way and then the other if we don't get the result I want (slow, steady, quiet, smooth idle).