Today turned out to be a big win. I pulled the cowling to get at the lower plugs. When I check spark plugs I first test them just as they came out of the airplane. I was looking for a misbehaving plug and I found one, bottom cylinder 4. It had no spark in the tester so it was the likely culprit. Interestingly, after I cleaned it up it seemed to be OK. But I decided to pitch it and install a brand new one. That did the trick. The runup was perfect and I am back in business. Today's photo is of the offending spark plug.