Jim Burns wrote:
PA32R-300 Lance with factory fresh Lycoming OH IO540-K.
Disclaimer: This is based on experience with fuel systems on ground
vehicles and equipment. I don't have an A&P; I don't even have a TG&Y.
Your mileage may vary.
Having said that, I like practical puzzles, so here goes...
Did this servo ever work right on this engine? I can't tell from your
post if you got the new engine, flew (and/or ran the engine on the
ground) for a while with no trouble, and then did the JPI install, or
if this all happened at once. If the servo never worked on this engine,
there is some fundamental problem - bad servo, bad engine. But if it
worked until the JPI install, then if you back out the JPI install far
enough, it should work again. It looks like you've taken some steps
towards backing out the install (like replacing the JPI transducer with
the original stock fuel line) and not gotten anywhere, so perhaps there
is a more fundamental problem.
This might sound silly, but have you checked the rigging of the throttle
and mixture controls? If the throttle lever on the servo was at idle
but the throttle control in the cockpit was set at 25% when the linkage
got reassembled, you'd never be able to set the servo to full throttle.
Idle is rough, engine wants to cut out unless you jockey the throttle.
In my extremely limited understanding of how a fuel servo works, it is
looking at both the setting of the manual controls AND the volume of air
being sucked in by the engine when it decides how much fuel to send out.
If the engine is getting "extra" air - air that didn't come through the
fuel servo - then it will run lean, because the servo isn't adding fuel
to compensate for the "extra" air. "Extra" air can come from leaky
gaskets, seals, O-rings between the fuel servo and the intake ports on
the heads. Maybe a crack in the intake plumbing, including one or more
of the heads? Valves not adjusted correctly (if applicable) or valve
timing wrong? (Wrong valve timing would only really apply if this engine
has never run correctly for you... this would imply that the camshaft
was not installed correctly, or the pushrods or rocker arms are the
wrong ones - things that could only have happened at engine assembly
time.)
Flowed fuel through servo, without the output hose on servo, no
pressure and barely any fuel moving through servo.
If the engine wasn't turning, this might not be weird. If the servo
doesn't "see" lots of air flowing, it won't send out much fuel. (I
think.) The official way to test this is probably to put the servo on
a calibrated flow bench and suck a known amount of air through it,
while maintaining input fuel pressure/volume at a certain level, and
measure the output pressure/volume. The unofficial way might involve a
shop vac, some duct tape, and fervent prayers that the motor brushes in
the shop vac don't spark *too* much, what with all that fuel vapor
around...
Could the ball valve which meters the servo's output be stuck limiting
the output?
I would think that if it was stuck really good, you'd get nothing at
all from the output hose of the servo. If there was a piece of crud in
the ball valve, that might explain what you're seeing, but how'd the
crud get in there?
Any other ideas?
Precision seems to have most of their service literature online
http://www.precisionairmotive.com/rsasupport.htm
http://www.precisionairmotive.com/rs...leshooting.htm
http://www.precisionairmotive.com/servpubs.htm
and their troubleshooting stuff seems to be very much geared to "tests
you can do at East Frog Toe Airport without disassembling the servo,
so you can figure out what's going on before you spend lots of money".
It might pay to take a look at it.
I hope this helps!
Matt Roberds
Disclaimer: This is based on experience with fuel systems on ground
vehicles and equipment. I don't have an A&P; I don't even have a TG&Y.
Your mileage may vary.