Figure this one out

Check out this Autolite 45. Factor in that this engine is supercharged. It is the only plug of 8 that appears this way. The other seven are evenly lightly sooted due to a fairly rich mixture and mostly easy mid-winter runs. I switched back to the 45's for the season when colder driving conditions and short hops caused the Autolite AR33's to foul on a regular basis.

I've never seen a plug sooted only on 1/2 the insulator up to a horizontal edge line. The back has a mirror image soot-limit edge line at about 180 degrees around.

I have an idea but wanted to pose a mind teaser to the rest of the forum. Also I could be totally wrong and one of the members could bring this anomily to light.

-Geary

I'm no expert.... but this may be what they call Splash Fouling?

Below from some good web sites

Mark

Splashed fouling, Small islands of contaminants on the insulator indicate splash deposits. Replace with new plugs of the correct heat range. The use of fuel additives, carburetor and choke cleaners or other aggressive solvents before installing new plugs is the most common cause of this condition. This may sometimes occur after a long-delayed tune-up. Here, deposits accumulated after long periods of misfiring or low power operation may be suddenly loosened when normal combustion temperatures are restored after a set of new plugs has been installed. During a high-speed run, these materials shedding off the piston are thrown against the hot insulator surfaces

Mark - That is a new plug with perhaps 200 miles. Without chokes and the big crossram, startup is interesting requiring more than usual priming and a stall or two until oil pressure is up and she's on the main primary jets. Short hops in and out of the garage will progessively foul the plugs so you end up running the car for a few minutes each time to clear excessive fuel and allow the plugs to burn off some accumulation. The supercharger setup is a great series of compromises and balancing acts. This plug stands out because the other 7 are moderately sooted all around as expected for a twenty minute 35 mph local run.

I'm starting to think that it is misfiring and is getting washed clean with pressurized fuel rich air from the adjacent intake valve.

As Spock once said: If you eliminate the impossible, whatever remains, however improbable, must be the truth.

- Geary

It could be the plug is fouling on either the intake or exhaust side of plug. Maybe an issue with valve lash settings. Have you ever tried centering the plugs. Mark on the side of plug where the bent arm attaches to plug and when tighting plugs, center it between the valve openings. Idea is to get opening to face largest portion of head chamber.
Bob