Ok I am a student who is in the process go getting his A&P License and
just finished the portion dealing with what your asking..

When you run your engine at full rich you are giving the engine more
fuel than it can fully burn and use to push the cylinder down and drive
the other cylinders back up compressing the fuel air mixture. It will
continue to push but you won't get the full potential from your
compression... A way to see if your running at the correct mixture and
see if your engine is functioning to its full potentail you should
perform a MAG drop check...

To do this... after you have run your engine up and have it warm...
enrichen your mixture until it just starts to run rough then pull back
slightly... then slowly pull your throttle back to around 1700 rpm then
switch briefly while watching your tachometer to your left then back to
both and then to right and then back to both... You should see a drop
of around 50 to 75 rpm drop if your mixture is set correctly.

Daniel Brooks
A&P Pending school completion


Daniel.
You are having an apples and oranges conversation here. We are dealing
with compression checks, not reduction in BMEP due to mixture changes.
You are correct about what you are describing, but we aren't talking about
that
subject now. We ARE looking for a reason to explain WHY rich mixtures
degrade
leakdown tests. With that understood, the reason lean cut numbers are
better than
rich cut numbers is entirely due to fuel washdown. We have to remember that
we are
talking about a very small volume of air actually moving thru the oriface of
the
differential pressure gauge. Rich mixtures wash the oil off the cylinder
walls a bit more
"efficiently" than lean mixtures. The oil film between the cylinder walls
and the rings
is the sealing mechanism, NOT the rings running ON THE WALLS. If you
diminish
oil content at the sealing surface by having excess fuel there, it is fairly
obvious that
ring "blowby" will increase. A second effect, but much smaller is the
effect of excess
fuel on exhaust deposits on the exhaust valve seat. Current thinking seems
to indicate that
fuel will "wet" the deposits and cause them to stick to each other a bit
preventing
radial movement of the valve into the center of the seat. The guides also
use an
oil film to seperate the guide from the valve stem, a slight reduction in
the oil film thickness
can allow the valve to "rattle" to a position other than the center of the
guide bore. Since we
are only talking tenths here, you can start to see wht there is so much
variability in one check
to another............ ANYWAY............. These are the main reasons rich
idling will adversely effect
leakdown test results. They are proven, not conjecture. Earlier I gave
reasons from allied
fields to make the case in a more united front, but for some reason I forgot
to mention that
I'm an A&P in the game since 1979 and this particular subject hasn't changed
over the years.

Scott