wrote:
B A R R Y wrote:

Do you taxi full rich?

We used to, which occasionally have us slightly low compression
readings on one or more cylinders in the hangar. The low cylinder
would move, as there was nothing really wrong. G We now lean
aggressively until we get to the runup area, and have never seen the
problem again.

Our home field is 250 MSL.

Can you (or anyone else) explain the mechanism of this? Why a full ruch
taxi would affect compression? I routinely lean the mixture for idle
and taxi because I've learned that at my home airport (KORL) the plugs
can foul between the ram and the run up area. But I don't understand
(and I'd like to be educated) how it affects compression.

John Stevens
PP-ASEL


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