My plugs came out black and sooty until I replaced the O2 sensor. Do a search on mixture adjustment and O2 sensor test. If your running rich and your O2 sensor is working properly the EHA will try to lean it out, but if it is mechanicaly adjusted way out of whack then the EHA can't compensate. If you are running rich and the EHA readings suggest that everything is OK then I would suspect an O2 sensor that while it may still be operating has slipped out of adjustment.

Jorg