I'm not sure what the previous owner did, but I had to replace my '07 340 Sundancers line in '16. It was a major PITA, but I haven't had odors since (or pump turning on intermittently due to loss of vacuum), after having had issues for years. Rub you...
I had this issue on an 8.1 Merc in a 340. But it was just a spark plug jacket that had popped off. Seems like you have checked that though. Other than that, altitude change? That's all I got.
