Just completed a restoration of a 66 hemi carburetor set and in reviewing carter manual and also the Service manual for setting spec the whole issue of setting idle and idle mixture of the pair become somewhat of an issue. Frankly the service manual is vague. Additionally as I examine the carb base, base gasket and intake manifold in detail the whole idle air flow pattern is pretty interesting.

66-67 hemi carbs are bypass air idle carbs with idle fuel mixture screws on both carbs. There is no throttle idle screw and blades are closed at idle. From all the various source info I read for these carb you should equally balance them at idle. So both contribute equally to idle.

You set idle air flow with the air by pass screw. You set idle fuel mixture with the idle mixture screws. This flows air from above the primary ports to the center of the carb and out the center of the carburetor. There are cut slots on the base of the carburetor as you can see on the rear of the primary chambers for the air to flow back to the primary bores. The idle fuel comes out the normal mixture port below the throttle blades.

So air and fuel arrive from different locations as opposed to a normal carb where air flows around the throttle blade and directly by the transfer slot and the idle fuel port.

But more interesting is the manifold cutout and the base gasket chosen. Because the base gasket is oval cut with no real restriction on the center chamber where idle air is directed, the thickness of the base gasket allows air to flow not only into the primary port via the cutout, but also into the secondary port of the manifold away from idle fuel. There is communication between the secondary and primary ports. The rear carb is the main carb and the primary port also is milled to provide communication between both sides of the manifold for the primary ports.

The front carb which progressively comes in has no such cutout on the manifold, but still has a partial communication based on the base gasket and the center chamber of the carburetor.

This makes me wonder why the engineers chose this base gasket as it seems the idle air and idle fuel are more separated especially when you consider how the normal 68 and up carbs work. It seems certain cylinders have a greater opportunity to be lean or rich depending on the proximity to the air flow or fuel flow. If the base gasket was of the normal 4 hole design like single carb set ups use the airflow at idle would be directed only to the primary ports where the fuel is.

Not sure if anyone has ever thought about this or tested other base gaskets. Would love to know what why the engineers went this route with the chosen base gasket.