I know this is an older post but it’s something that’s always coming up.
I see a couple of guys struggling for some time trying to get an engine running so that it would be air worthy. Finally my curiosity has me introduce myself and I find out its been sitting a few years but it ran great when it was put away, than ask all the stupid questions like “how old is the fuel?” I than ask “is that a new glow plug?” They usually say “no, but it starts and runs.” I say humor me and try a fresh one.
Problem solved.

I have even fallen in the same trap myself. I have a YS 45 RE that I fly regularly, one day it just won’t idle. I start tweaking with the pump screw and it just will not settle down.
Then I remembered, TRY ANOTHER PLUG dummy! That was it. Now I get to find the old settings.

Plugs can glow, look uniform and intact, (sometimes looks gray) but in fact they are finished.

The moral of the story is, if you have an engine that was running fine the last time it ran and now suddenly it won’t, try the simplest tune up first, a new glow plug.