Understanding fuel servo maintenance

wrote:
(followups set to r.a.owning)

In rec.aviation.owning Robert M. Gary wrote:
The fuel shop fixed my fuel servo by replacing the spring that holds
the fuel regulating diaphragm. The tech said the previous spring was
very stiff and he replaced it with one that was softer. Does that
indicate that the previous spring was broken (i.e. stiffer than it
should have been) or does Lycoming (Airmotive really) have a variety
of springs for adjustment purposes??

I don't know anything about fuel servos. But I have played with springs
before. For compression springs made out of a piece of wire that is
coiled up, the common failure modes seem to be: 1) breaking into pieces,
2) getting less springy over time - in other words, when new, a 10 lb.
force caused 0.25" deflection, but now a 10 lb. force causes 0.5"
deflection, and 3) taking a set - the springiness is about the same but
the free length is shorter than when new. I guess getting _more_
springy over time (10 lb = 0.25" new, 10 lb = 0.1" now) is possible but
I don't think it's very common.

Thanks!

On 11/21/06 09:23, Robert M. Gary wrote:

[ snip ]


I just need to know that its not going to need to be adjusted again
while I'm flying over the Sierras. I like to fly to some very remote
areas of Mexico. Taking a 2 week donkey ride back to the states through
the desert isn't appealing.

I hear you can ride on the tops of trains ;-\



-Robert

Jon Kraus wrote:
10 hours? And you said OK to that? I was hoping you were a little
smarter than that ;-)

We had our servo replaced at annual and were charged 2 hours for the
R&R.

Just curious, did you already have the lower cowl dropped during the
R&R? If you went through the lower cowl vs. removing the intake stacks
I usually get charged 3 hours just to drop and return the lower cowl. I
can usually do it myself but it takes me 4-5 hours myself.

-Robert

Originally Posted by Robert M. Gary

The fuel shop fixed my fuel servo by replacing the spring that holds
the fuel regulating diaphragm. The tech said the previous spring was
very stiff and he replaced it with one that was softer. Does that
indicate that the previous spring was broken (i.e. stiffer than it
should have been) or does Lycoming (Airmotive really) have a variety of
springs for adjustment purposes?? Did they fix the problem or just
adjust the fuel servo to work "post-problem".

-Robert, IO-360-A3B6

I just got my 2001 Cessna 172SP with 350 hours on it back from annual and paid 2K for a fuel servo overhaul noted as fuel stain around the control arm connectors. My A&P at a busy high end facility called Lycoming for an explanation for the reason for a gummed up low time Servo and was told it was overpriming. I never prime over 3 sec which is POH.The only performance issue was some subtle power loss but this issue can cause engine failure/shutdown. I am going to call the overhaul facility for their rendition of this but it is a very under reported issue and hard to diagnose short of an annual.I do not know about the spring issue but these servos obviously have some insidious degradation problems.Since your life depends on it, I would have it overhauled and then watch this carefully and maybe have some one pull it or look up into the throttle body with a scope at each annual to be sure there is no recurring gum up since this cannot be seen from outside til late in the game.