On a recent flight of about 6hrs, 1hr before landing, FL100, -5C, peak EGT, the engine started running a bit irregularly.
One can achieve this exact effect anytime by over-leaning, which is easily done if one pulls the mixture back too fast (top of climb, etc) but in this case the EGTs didn’t show anything. This was confirmed later with the EDM700 log.
I immediately went richer and it went away. Then it did something similar for another couple of seconds and after than, for the next hour, all perfect.
What could this be?
A google suggests this sort of thing is pretty common and nearly always remains unexplained. Candidates might be
The last three would show up in the EDM700 EGT data however.
I am doing a service on it shortly so will have a good look around, including inside the magneto.
Does this event pop up on the graph after uploading the data of the engine monitor?
No.
Only my subsequent leaning showed.
your headsets ANR or ears decompression?
2 people noticed it.
The oil had 35hrs on it so did a service on the plane.
Nothing was found, other that the magneto was, in the opinion of my mechanic, not clicking as well as it should on the impulse coupling. Mag timing was spot on however. I have a spare so that was installed. The mag drop has much improved, from borderline (meeting the spec only if leaned IAW the Lyco instructions) to a clean 120rpm. Something wasn’t right with the old mag, for sure. It makes a sort of scraping sound when rotated by hand.
Crap really, for maybe 200-300hrs time in service! The magneto which was installed today was the one mentioned here and that one went back to QAA for a very careful overhaul because they could not reproduce a problem. They evidently did a very good job on it. The two halves are synced within a small fraction of a degree, too. GA gets so many substandard components… alternators making a strange noise after a few hundred hours in service, etc. Luckily most of these things don’t actually fail, and probably most of it would remain un-noticed in normal GA ops.
I am still getting this occassionally, and changing the mag doesn’t change it. Happens on cold starts only; never on hot starts.
What that odd event was remains a mystery but it wasn’t related to fuel flow or (a total loss of) ignition because that would show up in the EGTs. OTOH since it lasted for less than the EDM sample interval (5 secs or so) it may not show in there anyway.
Did you put the fuel pump on, or just go rich?
Just went rich. There wasn’t enough time to really think about it, and the symptom was close to what you get if you over-lean.
Sounds like our Grob115E drill for a blocked nozzle (strong but uneven running)
Mixture…Rich
throttle….Full for 5 secs
Subsequently reset best power mixture and throttle setting
I’m guessing sample rate didn’t pick up on the data?
Interesting that somebody has an SOP for this 
Yes I think it took under the 5 seconds which is the EDM700 sample rate.
Peter wrote:
2 people noticed it.
Funny enough, I get ear decompression at same altitudes as my partner
Peter wrote:
On a recent flight of about 6hrs, 1hr before landing
Maybe it was eating some fuel dirt? The last hours may show some rough engine running either from dirt/water in fuel or simply fluid imbalance in the wing (unless fuel pump is ON or you have aerobatics fuel tanks)? tough if you had a dry run with air/fuel mix it is very noticeable
