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Just changed the plugs in my 71, 350, 270hp and I have found the plug on my #1 cylinder dry black and sooty. As seen in the picture plugs 3, 5, and 7 appear to be ok. Plugs 2, 4, 6 and 8 are consistent with 3, 5, and 7. I have done a bit a research and the heavy black soot appears to be either an overly rich condition or ring failure in cylinder #1. I have run a bore scope through all the cylinders and all are fairly consistent with some slight scoring. Approx 2k miles ago I lapped the valves and replaced the seals. I have a hard time believing one plug only, out of 8 would show evidence of a rich condition. Your thoughts?
Looking closely at your photo, that looks like oil on that plug. Rich is black but dry and sooty looking. Oil is shiny and wet Looking.
rings? bad valve guide?
Recommend you do a compression test on all cylinders. If #1 is borderline/low relative to the others, it could be a cracked ring. Per your post, I assume valves are good.
Recommend you do a compression test on all cylinders. If #1 is borderline/low relative to the others, it could be a cracked ring. Per your post, I assume valves are good.
Compression test is 130-140 across all the cylinders.
Just a quick update…..I changed out the plugs, checked on the resistance in the #1 wire(all good). Did a couple hundred miles with several hard accelerations. A check on the #1 plug shows no black deposits, all good. A check on the original black plug shows the resistance consistent with the other 7. So my stab at an uneducated guess says either the plug messes up when hot or I didn’t have the plug wire on properly. Thanks for everyone’s input and suggestions.
One Black Plug
