I'm flying my Europa Tri-gear motorglider with a 914 turbo with an
intercooler and a single mini Andair gasolator. I've had a problem with
rapid EGT peaking above 3800 rpm which, thank goodness, powers the MG up =
at
300 fpm. Needless to say, I've been doing a lot of sleuthing to fix =
this
condition. The CHT never went out the top.
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At first I thought it was the Andair. I cleaned it thoroughly but it =
didn't
resolve the problem. =20
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I checked the tank vent line from the top of the tank to where it hooks =
into
the tube extending out the top of the fuselage by blowing into the line =
and
listening for bubbles to burble up into the tank. The fuel sight line
enters the bottom of the port saddle right next to where the gas exits =
the
foot of the saddle. I took the 11 1/2 inch aluminum vent tube out and =
bent
it so that when reinstalled it touched the front top of the tank because =
as
the MG climbs in a nose up attitude the fuel level vacates the front of =
the
tank first so that's where the outside air should enter the tank. It =
didn't
help. I'd still have EGT readings within normal limits until the tach =
read
above 3700 rpms. Once above this level the EGTs on all four cylinders =
would
rapidly climb above 1500 and I'd throttle back.
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I though of changing the needle valve setting but I feared that would =
just
mask the problem even if it worked. I hadn't diagnosed it yet.
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I tried using aviation gas instead of auto gas. It didn't help. I
disconnected the intake fuel line at the engine and using the fuel pumps
hooked up in parallel, I began to drain the auto fuel so I could replace =
it
with aviation fuel and maybe get better cooling. But while emptying the
port tank I could hear burgling! There was no bugling as I drained the
starboard saddle! =20
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The sight tube enters the port saddle right where the fuel exits. I =
figured
that once the fuel flow reached a critical point the sight gauge fuel =
level
was sucked down until air entered the tank. At a high fuel flow rate =
these
bubbles, right next to the fuel line, were being sucked into the fuel =
line.
I disconnected the fuel tank vent at the top of the fuselage, put my =
finger
over the end to block air intake, ran up the engine and, sure enough, =
the
EGT peaked and...finally...I watched the fuel level in to sight gauge
disappear. How could this be?
=20
So I figured it had to be a block in the fuel tank vent tube installed =
at
the top of the fuselage. Because I couldn=92t imagine who it could get
plugged I hadn't checked it. I checked it now and found it totally =
plugged!
I have gone through extreme pains to make my MG as slippery as possible =
and
faired in the two vent tubes. The fuselage looks like a fish with a =
tiny
dorsal fin. Somehow I'd plugged it with a dab of epoxy! =20
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This explained the cause of my problem...or, to be even more logical, it
explained a problem but the problem could have another underlying =
problem.=20
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I'd had two unintended engine-off landings and, though I'm a glider =
pilot, I
didn't want another beyond my control.
=20
Throughout this process I've been communicating with Ron Paragoris and, =
Ron,
I thank you for you clear thinking and sound advice.
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I just order a second mini Andair. I'll install it in parallel.
=20
I've also ordered a UMA 1 =BC=94 Fuel/Airbox Pressure gauge with red =
markings at
2.1 psi and 5.0 psi with a green arc between. You can get one from Ed =
at
ed@umainstruments.com.=20
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The problem which may underlie the plugged tank vent is the problem of
less-than-adequate fuel pressure being overridden by the MAP in the =
airbox
being higher than the fuel pressure thus fuel starving the carburetor. =
The
UMA gauge shows the difference between the two in a little "steam =
gauge." =20
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Ron explained it to me well so(I hope you don=92t mind, Ron)here=92s his =
exact
words:
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Fluid seeks its own level. I think you may want to fool with a few soda
bottles to convince yourself what I say. Lets say we took a 2 liter soda
bottle and melted a 1/4" hole in the bottom of it. Lets fill it with =
water
and put the cap on it. It will not drain because atmosphere can not get =
in
to replace water. OK now lets drill a hole just below threads the size =
of a
drinking straw (corrugated to allow easy bend) or some soft copper or
aluminium, make this vent go up 1 or 2 feet. OK try experiment again, =
cover
vent and get no flow, finger off and get flow. OK you now have visual. =
OK
now lets put an extension on the bottle, lets say a garden hose or go to =
a
good hardware store and get something to extend up to height of the =
vent.
When you fill, of course the water will seek the same level in the fill =
as
in the vent, and when you experiment, you will find as long as air can =
get
into the vent, it will drain.
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I think you ran out of gas, I suspect restriction of fuel flow. I will
describe below that it is not fuel flow into a container that has =
ambient
pressure on it that you tested, but lack of flow into a container that =
is
pressurized to 40 inches of mercury, that you in fact did test, and your
plane failed the test.
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Please call John Hurst at Lockwood Aviation, and talk to him a bit on
differential pressure requirements of 914 if you can.
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Fuel flow on a 914 into an ambient container is pretty moot. The fuel =
flow
on a 914 could be 50 plus gallons with 2 pumps running in parallel on =
the
ground, less as per Rotax/new Europa.=20
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If you only have 1 GAS125, there is no way I would trust it to provide =
ample
fuel supply, I would not trust 2 GAS125s. Forget it if you have supplied
filters and a GAS125, too much restriction. If it worked once, it was =
only
an unbelievable minor clog away from no way.
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Did you read Rotax recommendation in Service manual that describes
differential requirements?
=20
OK 1 more time, when you are at take off power the airbox gets =
pressurized.
Unless you have 2 to 5 pounds more fuel pressure than airbox pressure, =
you
will run out of gas as you describe. At this point in time if you were =
to do
a simple test like see if the fuel pump will pump fuel into a container,
even with a restriction of course it will. That test is moot. Now if you
were to put that container under an absolute pressure of 40 inches of
mercury on the ground, well you best get at least what the engine is
drinking. I will repeat it below, you need more pressure to overcome
gravity, so when testing pitch nose up.
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If you want to perform that test, that will show what is really =
happening.
Yes I know the GAS125 is expensive and beautiful, ditch it. Remember =
when
you climb, nose high you need even more fuel pressure.=20
=20
If you must fly before you get a differential pressure gauge, put in the
supplied filters only. I would take off to a comfortable height at no =
more
than 100% power, then when you feel comfortable then fool with 115%, =
this
will stack running out of fuel at low altitude in your favor. Less power
even better if it does not comprimise flight.
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If you need absolute scientific convincing, place your bird nose up as =
in
climb, plumb the output of the fuel pressure regulator to a translucent =
or
glass container. It must be sealed, then take a manifold pressure gauge =
with
one of those hand pumps, or a bicycle pumps and keep pressure begin at =
40
inches absolute, you want to see at least 10 GPH at least to be safe, =
then
begin increasing pressure till you fall below the 10GPH. You may need a
relief valve, a fishtank style should be adequate for this test.=20
=20
Before you clean your GAS125 try that, then try the factory filters. If =
you
can't get at least 10GPH at 40 inches you will be run out of gas.
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You can even try same test in cruise attitude, climb attitude and tail
almost on ground, you will see for yourself in inches of mercury exact =
what
each attitude will yield.=20
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If you have an accurate MP gauge, if you kept the boost to 2 inches less
than whatever flowed 10 GPH you should be safe.
=20
Flow into ambient pressure is not much a test on a 914. On 912 or 912S =
it is
an OK test. You need to test against full power boost pressure in =
airbox,
and want 2 to 5 PSI over that.
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I am sorry for repeating myself, and circling the wagon and looking at =
it
---From different angles, but this is important.
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Differential gauge, redline at 2 PSI, go below redline and needle begins =
to
jump, probably 20 seconds from engine failure, reduce power right now to
where you have at least 2 PSI, I don't care if it is 100 FPM down, that =
is
max power you can maintain.=20
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On take off roll, if you are not in green abort take off.
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When I explained the on the ground mock test of pumping the output of =
the
fuel pressure regulator into a sealed container to simulate overcoming
airbox pressure, I forgot to mention you will need to connect the =
diaphram
to the 40 inch mercury pressure as well!
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The regulator is controlled by this diaphram, and it is critical on 914s
that this hose that goes from fuel pressure regulator to airbox is in
pristine shape, no leaks or restrictions.
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A real balancing act as you see. Once you understand (that very very few
folk do) it is not that difficult. Problem is many a folk working on the =
914
apply normal aspirated think to it, and it is different.
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Rob, again.
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So yesterday, after cleaning out the plugged tank vent I flew the MG and =
had
no problem with the EGT at high power and high nose up. Still I=92m =
going to
install a second mini Andair and the UMA pressure differential gauge.
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By the way, I=92ve uploaded pictures to the gallery of N128HW (prounced =
=93I too
ate hotel whiskey=94, hic up) in the air.
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Rob Neils
=20
Spokane WA
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