Yes there is a secondary idle circuit and transfer slot. On most traditional Holley 4 bbls, the secondary idle port is very restrictive and the t-slot is high enough that a small increase in secondary throttle position will not richen the mixture.
Secondary transfer slot positions vary, as do secondary idle ports.
Very generally they seem to be as follows:
Two idle mixture screws
Secondary transfer slots above the throttles. Vacuum secondaries further up than double pumpers.
Secondary idle ports very small, greatly restricting fuel at idle. Vertically they line up with the t-slot.
->exception: carbs using baseplate channel to connect a primary idle port to secondary idle port. Secondary idle ports are full size.
Four idle mixture screws
Secondary transfer slots and idle ports nearly mirror primary transfer and idle ports.
With a four corner idle mixture screws there are two fully operating idle systems. Therefore under the same conditions, with the same calibrations a four corner carb will deliver double the fuel at idle. On an engine with low manifold vacuum at idle, this can be very helpful. But I mention this mostly because some are surprised that the car idles well with the idle mixture screws hardly open. Well that's why and its nothing to worry about. As long as turning the mixture screws cause a change you know they are in their working range.
