Another quick thought, I have had it happen when the needle valve was not seating correctly or was dried out and would let more gas go by than needed and caused flooding and soot like you have. Also would drip into the manifold when sitting with the engine off. Rebuilt more carbs than I can remember. Just thought of this and I hope this will help you out. This is unlikely but a carb jet inside may be not tight and letting gas go by it or it could be cracked. Just trying to come up with some suggestions. H.
