After reading your posts, I feel your frustration. Is it a starvation problem or is it flooding problem? Without seeing or touching it, from what you say, it may be and likely is flooding? I have rebuilt quite a few carbs over the years, but have never had the Venture carbs apart.
One set of carbs that did stump me was on a Virago. I had them off about 4 times and could find nothing. Clean ports, set correctly, clean diaphram with no pin holes, and the floats... floated in the fuel. Out of frustration I replaced the needle and seats, even though I knew they weren't badly worn. What did catch my eye on my final carb assembly, was that there were slight wear marks on the side of one bowl. One float was hanging up! That was the problem. With a slight adjustment it was fixed!
I have seen many people replace floats which fixed their problem, but a good chance it was doing the same thing...hanging up or down on the bowl. Maybe both.
Now about the black stuff coming from your fuel bowls. If you say the inside of the fuel tank, screens and petcock are clean, then my only suggestion would be the fuel hose may be deteriorating or it isn't the proper fuel hose being used?
Sorry for jabbering on so long. Just some thoughts, but I really hope they can help.