Have you ever done an in-flight mag check? The folks at GAMI suggest
this all the time, as does John Deakin of Advanced Pilot Seminars.
If you have an engine management instrument like the JPI, EI or GEM
you can do this to check for a bad plug that wouldn't normally show
up. First you'll establish yourself in the flight, then lean the
engine to a good lean of peak setting, just before the engine starts
to stumble and continue the flight for a few minutes. Then do a mag
check and watch the CHT in each of the cylinders. If you have a bad
or even marginal plug, it'll show up here due to the stress of being
lean. The CHT will drop because simply the cylinder is not doing any
work. Don't be afraid of the engine stumbling during this process and
it usually will take about 1 minute or more on each mag. They do
report finding a few bad plugs right out of the box!

Good luck and keep informed of your findings.

On Sun, 08 Aug 2004 05:59:07 GMT, (John_F) wrote:

Most likely your problem is the plug fired by the Light speed system
is shorted out by a lead ball. If you are firing on only one plug
you only have one flame front instead of two so it takes longer to
burn across the cylinder. This effectively retards the timing on that
cylinder and the charge is still burning when the valve opens hence
the EGT reads hotter.
John

On 7 Aug 2004 18:40:55 -0700, (Jim Harper) wrote:

Was going to say woes, but really, just an issue.

Airplane is an RV-8A with an IO-360 (200 hp). Had some issues with
speed in the past and posted to rah, got some useful answers. We're
getting there on the speed. Ok, new issue.

Was flying to C-22 today. Long story short. As the flight progressed,
noticed some roughness which sort of resolved. No significant loss of
power...just a bit of a stumble, which I thought got better. In
retrospect, perhaps not.

Was high, and close to the field. A few minutes later, perusing the
engine gauges (four point EGT, four point CHT (and others)) I noted
that cylinder #4's EGT was significantly hi. I mean, 1500 degrees. All
the rest were within normal range. CHT's all in normal range. Really,
everything else normal. I richened the mixture considerably and the
temp dropped somewhat. Was on the descent anyway and so I reduced the
power considerably, and it looked better...down into the 1300 degrees
where I normally lean to.

Ok, landed the airplane. Taxiing to the ramp, is running fine. I
checked the right mag, which is a normal mag, lower plugs, and it was
normal...a little extra rough, like one sees, but fine. I then checked
the left mag...which is actually a lightspeed plazma ignition.
Generally when you check that, it is absolutely smooth. No "mag check
roughness" at all. Ok, I checked it this time and it WAS rough. Sort
of like one cylinder wasn't firing (wink, wink). At least it reminded
me of my old TR-3 when one of the cylinders wasn't firing. Just to
make sure that we weren't seeing a lightspeed failure, and a live
right mag, I pulled the circuit breaker on the lightspeed...engine
died...circuit breaker in and it runs, but rough.

I didn't have time to fiddle with it today, but I'm thinking that I
failed a lightspeed plug (which is a normal automotive plug) on
cylinder #4. Anyone disagree? I read somewhere that when one of the
plugs fail on a cylinder the EGT goes up due to unburned fuel burning
in the exhaust pipe.

I'll change the plug next time I get back to the airplane, but anyone
else have an alternative explanation?

Thanks!

Jim