Mixture screws fully closed. Cylinders still firing rich. wow. 3rd gen.

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Did you find blockages in the pilot air or pilot fuel circuit? Have you ever removed the pilot fuel jets and rodded clean the emulsion tube cross-drilled holes? That is where the pilot air mixes with the pilot fuel to form the emulsion mixture.

I hope you did rod out all the air jets--pilot, choke, transfers, and main.

And yes all those transfer ports need to be open, but the big question is what has been pulled thru the transfer air port to cause that blockage?

Whatever it is may also be blocking the mixture port from getting the necessary air.

What are you mixture screw settings now?

De-pairing wouldn't be the culprit unless you didn't seal the exhaust port side of the Pair system.

Maybe you can carefully and slowly spray carb cleaner with a straw tube thru each air jet and verify that flow is not blocked. Also do this for each fuel jet--basically you have to verify that each air and fuel circuit is free and clear and flowing as designed: pilot (idle mixture), pilot (transfer), choke enrichment and main. If you are going to clean a carburetor then you really must be thorough in all the circuits or it will never be right.