rough engine just after power reduction

Francois Marquis wrote:

Just for the record, absolute EGT temperature doesn't mean a thing if
you have not assessed its relationship to peak.

Of course

An unusual change in
EGT while cruising is a revelating event

I wasn't perusing the EGT in flight. More interested in putting
myself in a position to make the airport.

, 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)

Actually, no, it was significant. Normally during ground run before
shutdown (I follow the Lycoming SB for plug fouling, ground run at
~mag check rpm while lean), I see EGTs in the 1000s. That day they
were all higher -- closer to cruise EGT of about 1200-1300 with
#4 being 1400 and on R mag 1490.

Peak is about 1500 in my plane.

After cleaning the plugs, ground run EGTs back to normal. Still not
sure why fouled plugs = higher EGTs but I'll take it

Thanks
Sydney