I replaced the CDI box on the one engine in the plane and the problem disappeared. Of course the caps were replaced also. Then I stuck a piston at a show. Somehow, I had the thing running too lean in 105 Degree F ambient temps and lots of noise. Swapped in the spare engine and all was O.K.

I have not had the desire to replace the caps, yet. I did replace the piston, ring, cylinder, pin, bearing and clips. I'll probably get to the CDI's Caps this winter and give the two old CDIs another try in the spring. I typically fix in the winter, unless it is one of the show planes.

I also noticed that one has to be careful when installing the caps on the plug. If there is too much spring inside the cap once it is installed, the caps do not fully engage the plug's tip.