Airgap manifold you say? That means the plenum and carb are separated from the engine by an airspace like pontiac and Y-block ford but does yours have heat passages? Since the purpose of it for performance is to keep the mixture cooler and more dense-I would guess not.
My guess the engine water is up to normal temp but not underhood and manifold and carburetor, so the carb is off the choke too soon and we may even have some icing.I've seen carbs and manifold ice up outside at near 50 degrees F in high humidity. You installed a cold manifold and now we see the downside.Icing is worst at small throttle openings because the mixture expands so much across the closed throttle plates.
