[Rick Parker]

This is a fuel related problem. The floats are probably just a little too high and when the carb heat soaks after shutdown the fuel expands, percolates and drips out of the venturi boosters, and onto the throttle plates, probably the primary side. It drips past the throttle plates because they are slightly open (idle position). Then the moment you tip the throttle any additional fuel that has collected on the throttle plates and not evaporated pours into the plenum in raw liquid form and creats a massivly over rich condition that wets the plugs and takes several seconds of cranking to overcome. To verify this, bring it to full operating temp, shut it down and pull the air cleaner and watch for dripping fuel within a very few minutes. Take corrective measures. This can also happen with Webers. Fuel levels are critical and can play tricks on you.