Jay Honeck wrote:

I didn't have time to work on the plane today, but I'm gonna yank that plug
and have my mechanic test it in his brand new spark plug tester -- the kind
that actually pressurizes (or does it lower the pressure?) it while testing.

Increases...no matter what the ambient pressure in, the cylinder is many
times that inside.

Now if you're testing the ignition leads....

Of course you could bite the bullet and spend the $25 on a new plug.