EME 55 with about 5 tanks thru it installed in a 95" Howard Ike. Flight # 6 and it wouldn't start. Cranked it for about 15 minutes before giving up.

When I got it back to my shop I pulled the cowl, then the plug and attached it to the cap. When I rotated the crakshaft past the magnet, I got two sparks. One weak one from the tip of the electrode and a strong one from the hexagon base straight into the cap. I believe this is called leakage.

New plug tested in the cap produced a good spark at the electrode and then installed in the engine and it started right up as good as ever.

Has anyone ever experienced this before?

Not with a CM-6 plug. I've seen all sorts of unrelated failures with others though.

ahicks,

The plug came with the engine as a freebee. I was told it was anCM-6. It lasted 4 flights. It has a CM-6 now and no more problems. It was such a strange occurrence I thought it a good idea to tell the story.

Perhaps this video of a sparkplug factory in China might give you a clue. First thing we toss won't even run for the first time with them.

https://www.youtube.com/watch?v=is63QOaUkTw