My compression/cam calculations show the cranking cylinder pressure should be about 180 PSI at sea-level? This may be too much for pump gas? With Iron heads I would guess the engine is sensitive to detonation? You may not hear pinging, and a rich mixture and retarded ignition timming may be covering up the problem. That is why I suggested trying the E-85 Fuel, it is 105 octane and runs cooler if detonation is the problem, it should run better. The E-85 needs a richer mixture, but The carb jetting sounds rich to me, so it may be OK with the E-85. Anyhow, trying this test is cheap and easy.