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how is you compression on this one, compared to the others
Just did a check yesterday when I pulled the plugs: #1-78, 2-78, 3-77, 4-77. This has been consistent for my engine.
Not sure what is causing that, one cylinder looks like running too rich, or oil control ring bad.
I am pushing 700 hours on my 350 and plugs are still clean and compression is still 79/80 all the way around.
Are your CHTs consistent across all four? EGTs? Have you checked and adjusted your valves? Google has a lot of pictures of fouled plugs an suggested causes. No matter the cause you are very likely going to need to contact one of the two service centers sooner or later. Sooner is better. Ray Lawerence is very helpful and his number is 478 232-9560.