My experience has been that the carbs are often not the cause
of any given problem but can be fiddled with to hide a problem.
As the problem gets worse, the carbs are less and less able to
hide the problem and so it looks like the carbs are the problem
even though it is something else.

Look for vacuum leaks. Check the distributor and make sure
the timing stays the same at a given RPM; stuck advance weights
will give wacky behavior. My money is on vacuum leaks or blown
manifold / manifold gasket.
chris