flyer wrote:

I inadvertently did not return the ignition switch to "BOTH" after a
preflght mag check cause: distraction, I was an idiot) the other day
departing in my Glasair with my partner. The engine is a 150hp
O-320-E2D. The engine only developed 1900 rpm (2200 is normal for us
on 2 mags with fixed pitch prop) and climbed poorly and ran rough.
After figuring it out in approximately 1 minute, I switch back to
"BOTH" and everything was fine. That was a few months ago. Our
recent annual showed some low compression in #3 (55) and the others
were in the mid 60s, down a few points from last year. Not what I
would like but the engine runs strong. My partner said that the
single mag takeoff could have caused "detonation" or "cylinder
glazing" or "a bad flame front in the cylinders" and ruined the
compression.
I don't think that at that low power setting in a 7:1 compression
engine anything bad could happen. I am not aware of anything bad that
could happen in general from single mag operation at any power
setting, other than rough performance. This would be the same thing
as having a fouled plug in a cylinder, and nothing bad happens then.
Does anyone have any other feelings about this phenomenon.

I dunno. I am not a mechanic, and haven't flown in a long time. However, the
two most probable problems are that you could foul the plugs associated with the
inoperative magneto and that you would have the effect of a slightly retarded
spark--increased head temperature and egt and very slightly reduced power. The
real risk would be the lack of any automatic redundancy in the event the the
operating magneto shold fail. The short answer is that you probably didn't hurt
it any more on one mag than I did climbing out with the carb heat on after a
touch and go.

Try not to make my second, and much more dissastrous mistake as well: After
about the third time in two months that I forgot to turn off the carb heat on a
touch-and-go, I got the idea that about a two month break from flying would
enable me to forget the bad habbits which I had developed and resume the proper
and efficient use of my checklist. The dissastrous result was that two months
became three and three became six and ... and I currently hope to put flying
into next year's budget. What makes that bad decision so bad is that it
happened in 1983! :-(