Fuel servo output problem (long)

On May 7, 11:14 am, "Jim Burns" wrote:
Thanks Robert,
This should be a warranty claim, but the simplest thing to do would be to
simply swap the servo with a rebuilt. My real interest is what is happening
and why. I've been through Precision's website and online documents. The
only thing that makes sense to me is a stuck ball valve limiting the metered
fuel output. It's just about the only moving part within the fuel flow.
Jim

Ya. The fuel servo is a super simple system. As you mention, as long
as the diaphragm moves correctly, everything else is just physics.
When I sent mine in the fuel shop they seemed to have a low opinion of
the materials Precision has been using in their products since the
mid-90's. In any case, everyone seemed pretty surprised that a fuel
servo would go bad early. We never did find out what caused mine to
fail, but a full O/H fixed it. Hopefully your installation isn't as
tight as mine, I had to remove the exhaust tubes each time as well as
part of the lower cowl. Plus, after reintalling the servo, figure
several hours to get the idle settings correct.

BTW: When my servo would hang up I would get insane high fuel flows
from the JPI (like 13 gph at idle). In flight I would get high flows
too but when I tried to lean it out, it would eventually drop to zero
(making the anti-sweat fan stop). After the O/H I flew for 45 mintes
over the airport at various speeds and mixture settings, running LOP,
ROP, etc.


-robert

We're takeing a few steps back and looking at what's new vs what's old. We
checked the fuel pressure on the electric fuel pump, only 9 psi. Lycoming's
overhaul manual calls for the engine driven pump to push 18-26 psi. 9
doesn't sound like enough. We pulled the pump, opened it up and found
plenty of metal dust. It could be that the electric pump simply wasn't
pushing enough pressure against the diaphram and spring to open the ball
valve. Once we get a new electric pump installed and pressure checked we'll
know more.

Jim