This year's annual

wrote:

Can you (or anyone else) explain the mechanism of this? Why a full ruch
taxi would affect compression?

We were told that we could coke up valves, causing them to slightly
leak. Perhaps an A&P in this forum can explain more, or tell me why I'm
wrong (and we'll all learn G).

Leaning during taxi did cure the problem.

On Mon, 06 Nov 2006 15:59:25 +0000, B A R R Y wrote:

coke up valves

I'm guessing that this doesn't refer to the Pepsi competitor. So what
does "coke up" mean?

- Andrew

It's the heavy black carbon deposits that can build up on the valves,
piston crowns, ring grooves, etc... Nasty hard **** (technical term)

Jon

Andrew Gideon wrote:


I'm guessing that this doesn't refer to the Pepsi competitor. So what
does "coke up" mean?

- Andrew

I'm convinced compression readings vary with everything including
day-of-the-week and the phase of the moon etc.

My 172 M has (eventually) met compression for 31 years (I guess that
would be about 120 readings) but for about 10% of the readings it
required a retest 10 hours later to get a cylinder above 60, at which
point it would be back up. Through the years numbers though were
completely inconsistent except that the inconsistency was inconsistant
too.

I always used the same compression measuring rig (I called it the heart
failure model) so that was not the source of the variation.

John Thorpe preached that removing the upper spark plug before
compression testing could drop debris on any open valve faces reducing
the reading.