Jay Honeck wrote:

We've got the JPI EDM-700 on our Lycoming O-540, and love it.

In fact, I'm in the middle of diagnosing an engine problem that the JPI has
made much simpler to correct. The #2 cylinder is not always firing, but
only when set to the left mag.

Our ONLY indication of trouble was when the bar graph shot up to 1650
degrees (EGT), and the CHT dropped to 200. The engine never changed sound,
or missed a beat. (Well, until we turned the mags to "left" instead of
"both".)

Without the JPI, we wouldn't have known we had a problem.

Which is why I bought the thing in the first place! In my opinion, it has
paid for itself.

And if it turns out that the plug was fouled the analyzer didn't do
anything but make you nervous.