An air leak would lean the mixture. The problem was the mixture went rich. This was the third flight of the morning so the engine was well warmed up. Also, it was quite an abrupt change while in level flight. Hard to believe that it could have been leaking for a couple of years, then suddenly tightened up.

Not trying to be difficult as I can't figure it out myself but, as a "retired" reliability consultant, I look at "If it is this, then that."