I think your plugs show an over rich condition. The idle mix screw starting position should be 1 turn out. You tune for best running starting at 1 turn. This according to the Bing manual.
Overly rich can be caused by floats that don't float. No mater what they are set at if they are sinkers instead of floaters the gas will not be regulated. You can try floating them in some gasoline to see if they look like they have enough buoyancy but this is a judgement call. You can weight them but you need a good set to compare against. I change mine every few years. BTW several people on this list tel me that's too anal.
The jets and needles in the carbs also are wear items. Some don't see this point but as the needle goes up and down in the jet it rubs. It wears. You might consider at this point a complete carb rebuild. If your carbs are flat tops the diaphragms are rather expensive, they aren't cheap for the other carbs too, I try to keep the diaphragms as long as I can.