A half or 3/4 turn too rich on a Weber idle volume screw can indeed result in BLACK plugs, sooty exhaust, and acrid unburned-fuel smell in the cabin.
I started with 60 idle jets, but ended up with 57.5 a few years later. I got it pretty good and NOT too rich with the 60s, and with no stinky fumes.
I know this because I didn't know how to properly adjust the carbs when I first got my car. Fuel pressure was exactly 3 psi and float valves were in perfect condition and float height was spot-on.