18MM plugs were only used on the early OHV V-8 engines- - - -mid-1950's 239, 272, 292,312, etc. Flatheads used 14MM plugs.
Black and sooty is too rich- - - -start looking for a vacuum leak near the center cylinders that is causing the power valve on the carburetor to open up and flood the corner cylinders, while compensating for a lean condition near the center. Use a Propane torch with a piece of rubber tubing on the snorkle instead of the flame nozzle, or an acetyline torch with a little fuel gas flowing (NOT LIT!) and run the gas around the intake-to-block gasket with the engine running at a fast idle. If you find a leak, the RPM will go up and the miss, if any, will smooth out. That long, skinny intake manifold can be warped near the middle. Draw-file it or mill it flat if necessary and be sure to torque the bolts when you install it. It's also possible to get an internal leak on that engine- - -from the crankcase to the intake, so run just a little gas into the oil filler tube. Don't overdo that one- - - -we don't want a crankcase explosion!
Jerry
