John Galban wrote:

Puzzling! It sounds like a bad or fouled plug, but the EGT doesn't
make sense. If you were running on both mags, a bad plug would show
up as a higher EGT on that cylinder.

And it was higher, a bit, on a cylinder which usually isn't
the hottest...

If you switched to the rough
running mag, the EGT for the bad plug should drop.

'swhat I would have thought too.

Well, there was a lot of lead fouling in all the bottom plugs
(which were cleaned recently -- ugh!). But nothing that looked
as though it should have put one plug out of commission. Unless
I burnt it off with ground running, I'm suspicious that this
isn't the end of the problem.

Just pulling through the prop, it's clear that #4 cylinder
(the one w/ the high EGT) doesn't have as much compression
as the others. Still got compression though. Dunno how
quantitative. Pulled the rocker box cover, no sign of soot
in the oil (the poor man's "exhaust valve going bad" warning
system). #4 happens to be our only remaining NuChrome cylinder
and had lower compression than I'd like last annual (no leakage
past the valve though, and the wobble check was good)

How does one check the ignition wires for proper function,
and if one plug isn't producing spark as George suggested,
would it be fouled?

Methinks a compression check is in our future. Meworries about
another sort of check.

Meworries more about not tracking down the problem...of course
some people would say I worry too much about 1 minute of rough
running.

Cheers,
Sydney