How did you "clean the plugs"? Unless you have some sort of abrasive blaster, the areas that need to be cleaned can't be reached effectively. Back in ther dark ages (1950s) Champion spark plug company sold a sandblasting machine that used high pressure air and fine grit abrasive to blow away carbon deposits from the plug shell and the porcelain insulator. I haven's seen one of those machines in use in at least the past 40 years. If you happen to have access to a a glass bead blasting cabinet, it can be used to accomplish the cleaning procedure. Back when I was crew chief on a racing team, we kept several sets of spark plugs on hand, cleaned them up every week, and ran the same 40 or 50 plugs all season. It was not uncommon to go through 3 or 4 sets of plugs while adjusting the carburetor mixture to atmospheric conditions during practice before a night's racing.
Jerry
