I have followed this thread for some time. It just dawned on me you could have an idle circuit that is dead. My AFB four barrel car developed a knock at idle out of the blue. I was stunned as the engine had been fine for a long time. The knock was loud enough to be very disconcerting. After much fiddling I determined that one circuit of the carb was not functioning. You know this when a mixture screw does nothing. I swapped a different carb on there and presto, the knock was gone. To be frank, I was bloody amazed. After all these years you think you have been there and seen everything related to rogue engine problems. Not so. Keep on learning.
Another give away to non functioning or way too lean idle mixture is that you have to turn the idle speed screw way in to keep the thing running. To such a point that the venturies start to drip fuel to run the engine. When you see the dripping venturies you think the floats are too high or the needle and seat is not working. When you eliminate that probable cause, you are back to non functioning idle circuits or ones that are too lean for the engine combo. Been there done that on my new tripower set up. Just drilled the air feed and fuel feed locations on the idle circuit but have not lit it off yet.
You also need to be sure your throttle linkage has enough travel to enable the butterflies to fully close. If you do not, the carb will stay on part throttle operation and not go onto idle circuit.
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