(clarifying: put a thumb over the end of the PCV valve, not on the valve cover)
…and since you mention the carburetor boiling over: by all means make sure the manifold heat control valve is working; verify the carb float adjustment, and do the do the
fuel line mod, but you're reminding me of a case of an intake manifold with a crack in the plenum floor. This allowed exhaust into the intake tract—more of it when the engine was cold, less of it when the manifold heated up and pushed the edges of the crack together. Seen more often in certain aluminum intakes, but the case I'm remembering was an iron one. Caused symptoms like you describe.
