I find the phenomenon interesting and always read these posts about it, and I as curious as anyone on what causes it. It might happen to me some day and I want to know what causes it and what to do about it. I think I read at one time you can take a pint or new xacto knife and knock the taters off? If it comes off, it makes me wonder if it's some kind of ash??
They are usually smooth black lumps. Scrape them, and the black is just on the surface, and underneath is solid tan mass, like melted tan plastic. They can be scraped and broken off the wire, then the plug "works" again.
We never had this issue until we got PA61s and larger. It varies from engine to engine, one engine might start losing significant power after as little as 15-20 flights, others might go 50, even with exactly the same setup. This seems to do with just the size of the engine and the fact that it runs much hotter as it gets bigger, not particularly an issue with PAs. I would expect the same if I switched to SIG on the Jett engine.
We solved this by switching from SIG to Powermaster. We have never had a significant tater with Powermaster fuel of any variety. Randy says the problem is in the castor oil. I don't know for sure, but I was concerned about the "anti-foaming" ingredients in the SIG, which I presume is silicone. Bottom line, don't use SIG on your 61 and up stunt plane, or buy a lot of glow plugs.
Brett