Sydney,

Why don't you swap the top an bottom plugs, if they both look fine. The
next time it happens, and IF it is on a different mag, then you know for
sure this is a plug gone bad, and you'll know which one. If the same
mag, suspect wiring or a sticky valve. A can of Rislone in the oil can
solve it if it is minor. If not, the valve guide needs a reamer shot
and this is not a minor procedure. Guess you better be sure you are on
the right culprit first.

Just for the record, absolute EGT temperature doesn't mean a thing if
you have not assessed its relationship to peak. An unusual change in
EGT while cruising is a revelating event, but a power setting change
can (and will) change it too (particularely on carburated engine). If
your original comment that EGT#4 read higher, was it a "not the usual
reading I see" or "while it got rough, I saw it change". The first one
is normal (well, you didn't quantify the change, but I assume here it
was minimal), since on a power reduction you don't necessarely see the
same charge distribution across every cylinders (particularely if your
intakes aren't tuned). The other case is more revealing but symptomatic
of either a valve problem or a plug problem (so it just confirms you
that you were not dreaming about the rougness).

Trouble with those intermitents is that they are!

Francois (also get worried when his engine goes rough) Marquis

Sydney Hoeltzli wrote:
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