APT adjustment isn't the issue. What did the carb come off of originally? Barring any vacuum leaks, you're prolly going to find the idle bypass air orifices are too large for the 4.3L V6- the bypass acts like a controlled vacuum leak. If partially closing the choke blade or holding your hand over the primary side causes the idle to drop, then there's your answer. See what size they are now and close them accordingly. Again- no vacuum leaks!
Obviously you need to be real sure the choke/fast idle adjustment/cam/linkage isn't causing the primary shaft to hang open.
Has this carb ever run correctly? Warped castings or mismatched/torn gaskets can allow in too much unmetered air.
Last edited by cobalt327; 01-13-2013 at 10:02 PM.
Reason: Add choke info.