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Just for a bit more clarification... this on avgas (100LL), or cargas?
I'll give you one more data point that might be somewhat related. I've
noticed on my O-360 180 hp 8.5:1 Lycoming with the 91 octane cargas STC
that with a hot (recently flown) engine, it will actually diesel when I
cut the mags off. I was at idle, and rather than cut the mixture, I cut
the mags. It slowly shook to close to a stop, but at 100 rpm or so, it
continued to sputter and run enough to keep shaking and running. I was
very surprised at this, but when I started thinking about it, I realized
that at this (very) low RPM, there was lots of time to get the mixture
burning in the jugs, and also the idle throttle position allowed the MP to
build up to 20" or so (again very small airflow). It's almost like
lugging the engine at full throttle at 100 RPM... pretty tough.

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I was present when one of the mechanics, who I respect a lot, told a friend at
FXE that mogas and avgas have significantly different viscosities; and that an
engine will run much richer on one than the other. Me recollection fails me,
but your account indicates that magas has the lower viscosity and will run
rich. A rich mixture at idle, and carbon deposits from rich operation, used to
be the classic causes of "dieseling" in cars--sometimes people had to kill the
engines with the clutch! :-(

My best recollection of the mechanic's advice was to only use mogas if you could
use it all of the time. It might be possible to eliminate the problem by
leaning under all permissible modes of operation; and I plan to explore that
avenue when I have a plane avaible and start flying again.

Peter