is more than 10 to 15% you can bet there is a burnt valve or broken ring
which can be confirmed audibly during the test. If you are using an
aircraft testor, it would have a larger orifice which I would think
should indicate less leakage readings not more. Consistant 50's and 60's
wouldn't bother me but one low one is a problem. I do have to be careful
about leakage at the spark plug hole with my gauge and crankshaft
positioning is tricky. If you rotate the crank even a tiny bit past TDC
- the ring leakage skyrockets as the ring lifts off the lower side of
the piston groove!!! Be careful about that. I drop a stiff wire in each
cylinder as I come to it, rotate to TDC and mark it, back off a few
degrees, then rotate forward to just BEFORE TDC. Sounds like you've
already checked valve adjustment/clearance. If you can hear more leakage
from the low cylinder then you've already confirmed a problem I mostly
play with 4 cylinder liquid cooled engines. I don't use anything but
stock parts and simply don't see the break in phenomenon of improving
compression that air cooled engines have. So as a rule, I only use the
leakage testor to confirm and locate a suspected problem on an engine
that idles poorly or has indicated a problem with a compression check.
It is usually audibly obvious during a leakage test if you have one
cylinder leaking more than the others.
I'd suggest confirming the leakage with a compression gauge if you don't
trust the leakage gauge. The old trick of adding a squirt of oil and
retesting will still raise the compression reading substantially if the
rings are worn or not seated.
If you have a bad cylinder you need to find out why. The same assembly
or part defect may well affect all the cylinders in time. The good news
is - if you are not already certain there is a problem then there is a
good chance it is simply operator error or test equipment defects such
as a leaking hose.
Ken
Mike Kimball wrote:
I started my first annual inspection recently. I performed a differential
compression check on all cylinders and found an interesting thing and a
worrisome thing.
Interesting thing: On 7 cylinders I was getting only high 50s to low 60s
(over 80). It is my believe (for various reasons, not the least of which is
how well the engine is running) that these cylinders are all fine. I can't
figure out why the readings are so low. I can hear leaking air which I
believe is leaking into the case, not the intake or exhaust, i.e., past the
rings. As I'm sure everyone is aware, we like to see numbers above 70 and
all cylinders fairly close in readings. There is only about a dozen hours
of run time since it was rebuilt so far but I would have thought the rings
would have been set in by that amount of time. There's a little less than 6
hours in flight so the engine has seen continuous high power settings as
well.
Cylinder number 8 is only giving me about 32/80 and there's noticable
leaking air sounds through the exhaust pipe. Damn. Apparently, the exhaust
valve is not sealing. I wiggled the rocker and it's loose at TDC for the
measurement. I don't know enough about the characteristics of breaking in a
new engine so I thought I'd ping you guys on the list that do know. Is
there any chance that #8 will simply improve with more run time? Am I
looking at pulling that head so the valve can be lapped in or replaced?
(Man, I hope not.) The engine is running so well, it's tempting to just
call it a 7 cylinder engine and take the horsepower loss. My patience with
drawbacks is getting very thin.
Mike
044SR
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