Before I get serious and start tearing things apart, I wanted to pass this by you guys. Rebuilt flathead has run perfectly for Three years now. Installed taller banjo gearing this spring. Other than that there have been Absolutely no electrical or mechanical changes made to anything on this setup. MSD ignition, Running the two outer bank Strombergs on this manifold. Using NGK B6L plugs. set at .025 all the way up to .040. Symptoms. The one plug fouls out within 10-15 miles after installing the new plug or cleaning the old plug. Plug seems more like dry carbon vs oily carbon. Most of the time, the miss is only evident during moderate acceleration. Not during high rpm acceleration. All the other 7 plugs are a perfect tan color. Maybe another clue??? The left bank ( the location of this fouling) slightly backfires through the exhaust during deceleration. The right bank rarely does this. The Number 5 cylinder is the only location of this fouling. What I've tried thus far. B6L plugs. set at .025 all the way up to .040. Replaced the spark plug wire from distributor to plug. Slightly leaned out the idle circuit only on the front carb. What I haven't done yet. Tested the new spark plug wire resistance. Advanced the timing Pull the cap and check for cracks or wear Tested for a vacuum leak. Performed a compression or leak down test. Am I missing something here?? If I find the cap is good , Could the MSD somehow be weak at one cylinder ? Frustrating. Especially since this engines been running perfectly since its installation.