Based on your description of the plugs, it sounds like you have a rich running problem?? You can check engine vacuum at either an intake port or at the carb, but make sure the carb port is in the base plate. Looking for 20 inches or so of vacuum at idle. Based on the drivability issue it sounds like a lean misfire which doesn't account for the plug description. How old is that quadrajet? The primary throttle shafts will case a vacuum when they get some miles on them. Maybe time for a rebuild? They don't stay very happy either when they just sit around for a year or so. I'd point a finger at the fuel system first.
