Originally Posted by Happy Wanderer
I am not an expert on engine fueling (that's your dept) but from what I have read I believe the correct answer is heat. Overly rich mixtures may cause unspent fuel to enter into the exhaust flow. When this hits the cataclysmic perverter it causes super heating. This can lead to melting of the honeycomb structure and the metals (catalyst) it is made of.
Old enough to know better....
Young enough to try it again