On Oct 9, 9:50 am, wrote:

Interesting thought. I'm still skeptical that could be it, though.
I've timed the mags a few
different times over the course of the year or so it's occasionally happened. I'm pretty anal about
getting them dead-nuts on each other and exactly at 25 degrees. I don't want them advanced at all since
I run autogas, but don't want to detune them either.

Also, I've done the mag drop in flight when I suspected that it was happening. Everything was
fine... power smoothly decreased as expected. The "blow-out" thing could still be doing it, but I
wouldn't think it would only do it for the first 60 seconds of flight occasionally. It should be more
consistent or reproducable with higher MP (lower altitude takeoffs), etc.

Another idea. We had a Cessna 150 that used to lose some
power on takeoff, sometimes shuddered while doing it, and after a lot
of messing around we found that the valve stem/guide clearance was too
small. This was a problem is the smaller Continentals and when the
rebuilders did those engines they had to be careful not to get the
guides too tight. Lycomings tend to wear their guides bigger, so the
clearances get worse, but a few have had sticking valves caused by
carbon buildup on the exhaust valve stems. The sticking would usually
cause a power loss with vibration, and if they stuck bad enough the
pushrod would bend, and bend the rod tube like so:
http://www.prime-mover.org/Engines/L...r/bentpush.jpg
You said you'd done the wobble test and the guides were
worn but within limits. None were tight, huh? The 150's sticking
valves were the intakes, not the exhausts, and the Lycoming wobble
check applies only to the exhaust valves.

Dan