Does that necessarily mean that the plug is good?
Not always....the indicator light or the little meter on
the glow starter will show you that you still have
continuity ( the little coil wire is not broken ), and
current flow through the wire. It might appear to be
glowing red at a glance, but the engine may still not
run right.
Sometimes during a lean run....the little coil inside
the plug will overheat, and will "warp" out of shape.
That part of the warped ( or bent ) coil will be touching
the inside of the plug, and will not be glowing red.
That means your only getting about half the heat out
of the plug that's needed to run the engine properly.
If your motor isn't running properly....kinda spitting
and sputtering....won't idle for more that a few seconds,
dies out in flight for no reason.....have a real close look at
that plug and see if the coil is still nice and round. If
it looks like it's bent, hook the glow to it and see if
it's glowing red from top to bottom.
Hope this helps....Dave.