I've been wondering about this. Why in our engines is a fouled plug only useful for the trash bin while in an aircraft engine (full scale) we clean them all the time. Every annual they get pulled and cleaned with good results, they even sell reconditioned plugs to boot.

I've also cleaned them in my car and lawn mowers with no problems, and yes, they test good in a Champion spark plug tester, you know the kind that cleans them and also puts them under pressure to simulate a combustion chamber. You will find the occasional plug that fails that test under pressure, but would spark normally without being pressurized.

I've never tested the smaller plugs from weed eaters though, and that's the reason I ask why is it ok to clean all those others and not the small weed eater plugs? What's different between them?