When rubing your finger on the inside of the tailpipe this is what I would suspect.
A little dry grey residue on the inside of the exhaust is probably too lean.
Light dry black is probably pretty good.
Dry black is going to be too rich at the carb.
Wet black is oil.
I would "assume" that you are running rich with dual carbs and your discription of "black soot".
What do the plugs look like? I would use that as a more reliable indicator.
