Help needed for diagnosis - pictures inside
Am I the first to notice that the 2 spark plugs in question also have 'dirtier' looking threads on them? I had a problem with my spark plugs seating on my 408. I could actually see soot on the ends of my spark plug boots. At first I thought it was coming off of the exhaust but when I looked closely I could see that it was only the plugs (and the end of my spark plug boots) themselves that showed soot.
View attachment 1714760611
I switched to a tapered plug design and the problem (which was similar to yours) went away.
Some of my plugs had no issues, but I had a few that would foul. I started out running ACCEL Copper Core 784 plugs and switched to
CHAMPION Copper PlusRS12YC.
I will take a look at that.
