Maybe plugs go bad after being introduced to fuel... there's a lot of physics and chemistry that happen with these little engines that we don't realize sometimes.
There is platinum coating on the plug coil (or something like that) that has a catilytic reaction with the methanal in the fuel. As the coating wears, the plug gets weaker. Maybe after sitting for awhile, the plugs coating goes bad since it had fuel/exhaust exposed to it. Exhaust is acidic if I'm not mistaken.
Might be something worth experimenting... take a used plug that works today in the engine, throw it in a baggie for 6 months. Take another used plug that works in the engine, rinse it with DA and store it in a different baggie for 6 months. Take a new plug and store in a baggie for 6 months. See if you have any reactions from storage.
I have my first original glow plug that I ever had from 5 years ago. It quit working around 3 gallons. But, it still fires up bright red when on an ignitor. It came out of a Mach 15 from my xxx-nt.
I use it now to completely discharge my heater cells before charging them back up. It sits on the heater for 20 minutes or longer depending on the charge of the cell. I do this about once a month to cycle the cells. If I know the cell has a decent amount of charge in it, I put the plug in, then stand it in the refrigerator to keep the cell cool as it discharges. Otherwise it gets pretty hot due to the long amount of time its "dead shorting" with the glow plug.
I have the same two cells that I started this hobby with 5 years ago... they obviously last a long friggen time!
