If the idle screws have no effect there has to be another reason for it just as Vince said. A vaccum leak or fuel puddling. Years ago I had one that had to much fuel pressure, and it was flooding out of the idle passages so you couldn`t see it, and it was very difficult to figure out, but what gave it away was, after I would shut the car off it would continue to smoke out of the carb for about the next 10 minutes, that white hazy looking mist would rise out of the carb for a long period, that`s what told me something was amiss.
What is the fuel pressure? have you made any changes to the fuel system? is the vapour line still open that goes to the fuel tank? is the carbon canister still in use?