Weird /6 BBD issue (internal flooding?)

You’re barking up the wrong tree with the gasket idea. The screw you mention is for idle FUEL mixture. Which is air and gas. What typically happens is the Idle feed restrictors are sized too small for today’s fuel. That is why it wants the screw backed out all the way. When this fails to run the engine correctly, the mechanic will often turn the idle speed screw in until the carb idles off the main circuit. Also if your timing is say 8* btc or later you will amplify the problem significantly.