Citabria Fuel-Flow Problem

The A/C in question is a 1974 Bellanca 7ECA, re-engined with an O-320
so it's effectively a 7GCAA. It belongs to the Offutt AFB Aero Club
and has seen very little use, often sitting for weeks at a time until
a week ago when I began taking dual in it 2-3 times a week to get my
tailwheel endorsement. The fuel system is not for inverted flight, so
no header tank -- two 18-gallon wing tanks with check valves to a
single vent on the left, crossflow pipes to the aft fuel manifold
under the baggage compartment and crossflow pipes to the forward fuel
manifold on the firewall. Caps are non-vented and the gaskets are
like new.

This A/C is drawing 3-4 times as much fuel from the left tank as from
the right and it's not crossflowing on the ground, even overnight.
We've made right patterns as well as lefts -- makes no difference. We
haven't done any extended S&L flight, just 11 NM to Plattsmouth, NE
for pattern work. The club's A&P has blown out the feed and vent
lines and got a good blast into the right tank from both. We're
getting a free flow of clean gas out of all four drains.

So what's our next move? Anybody had this problem?

On Wed, 7 May 2008 11:17:24 -0700 (PDT), quietguy
wrote in
:

The fuel system is not for inverted flight, so
no header tank -- two 18-gallon wing tanks with check valves to a
single vent on the left, crossflow pipes to the aft fuel manifold
under the baggage compartment and crossflow pipes to the forward fuel
manifold on the firewall. Caps are non-vented and the gaskets are
like new.

This A/C is drawing 3-4 times as much fuel from the left tank as from
the right and it's not crossflowing on the ground, even overnight.
We've made right patterns as well as lefts -- makes no difference. We
haven't done any extended S&L flight, just 11 NM to Plattsmouth, NE
for pattern work. The club's A&P has blown out the feed and vent
lines and got a good blast into the right tank from both. We're
getting a free flow of clean gas out of all four drains.


I'm not able to accurately visualize the fuel system piping from your
description, but has the A&P checked for a stuck check valve in the
vent piping?

On May 8, 6:17*am, quietguy wrote:


This A/C is drawing 3-4 times as much fuel from the left tank as from
the right and it's not crossflowing on the ground, even overnight.

The lack of crossfeed sounds like the cross feed system is the
problem. Is the crossfeed supplied by the fuel selector being set at
both or a separate cross feed pipe? If you've had water sit in a line
for a long time some corrosion may have gunked up the line from the
right tank -I've seen that in ally pipes connected to a valve with
steel and brass parts.

Cheers

On May 7, 3:39 pm, WingFlaps wrote:

The lack of crossfeed sounds like the cross feed system is the
problem. Is the crossfeed supplied by the fuel selector being set at
both or a separate cross feed pipe? If you've had water sit in a line
for a long time some corrosion may have gunked up the line from the
right tank -I've seen that in ally pipes connected to a valve with
steel and brass parts.

There is no "crossfeed" system in a Citabria, but it will
crossflow through the line tees, Crossfeed systems are found in
multis.
There are two tanks, each having two outlets, one near the
front and one near the rear. The two front lines run down the
structural tubing below the front spar and meet at a tee above the
shutoff valve. The two rear lines run back and down behind the cabin
and tee together at the bottom of the airplane, and a single line runs
from there forward to join up with the teed-together front lines. The
fuel proceeds through that point through the shutoff and thence to the
strainer.
The vent is located under the left wing, and is plumbed into
the outboard end of the left tank. The inboard end of the left tank
has a vent fitting that is connected across the top of the cockpit to
the right tank with no check valve between, so that the right tank is
vented to the left and the left is vented outside. Everything will
work well as long as the fuel caps aren't leaking or have been
mistakenly replaced with vented caps, which will screw up the tank
pressures and cause uneven flow.
The OP says that all vent and fuel lines were blown clear and
were flowing OK. Yet the thing won't feed evenly or crossflow even
overnight, which has me puzzled mightily.
We have a 7ECA and a 7GCBC, both with this simple system, and
the only hassle I've ever had with either was an uneven flow problem
in the ECA caused by a vented cap that a previous owner had installed
on the RH tank, and in the GCBC when the LH cap gasket split and
sucked lots of fuel overboard.
I would suspect, at the least, that in the OP's airplane some of
the short lengths of rubber hose that join the various bits of tubing
are collapsed internally, allowing air to blow through when checking
and leaving the impression the the lines are clear, but collapse and
restrict the flow the rest of the time. Those hoses should be replaced
every five years to avoid the deterioration that comes with age and
especially with Mogas. The hose will look fine on the outside but the
inner liner can swell shut and restrict the flow. Asthmatic hoses.

Dan

quietguy wrote in news:b8a07cf2-f01e-4dba-9793-
:

The A/C in question is a 1974 Bellanca 7ECA, re-engined with an O-320
so it's effectively a 7GCAA. It belongs to the Offutt AFB Aero Club
and has seen very little use, often sitting for weeks at a time until
a week ago when I began taking dual in it 2-3 times a week to get my
tailwheel endorsement. The fuel system is not for inverted flight, so
no header tank -- two 18-gallon wing tanks with check valves to a
single vent on the left, crossflow pipes to the aft fuel manifold
under the baggage compartment and crossflow pipes to the forward fuel
manifold on the firewall. Caps are non-vented and the gaskets are
like new.

This A/C is drawing 3-4 times as much fuel from the left tank as from
the right and it's not crossflowing on the ground, even overnight.
We've made right patterns as well as lefts -- makes no difference. We
haven't done any extended S&L flight, just 11 NM to Plattsmouth, NE
for pattern work. The club's A&P has blown out the feed and vent
lines and got a good blast into the right tank from both. We're
getting a free flow of clean gas out of all four drains.

So what's our next move? Anybody had this problem?


OK, talked to my guy. He had several suggestions. One is to drain it all
dry and then blow some air (carefully!) into the right tank and see if
you can hear it coming across into the left tank by listening at the
filler cap.
Possible problems he came up with were a crushed or swollen fuel line
downstream of where you tested previously, or, he thought more likely,
incorrect placement of the little plastic fuel vent deflector and/or the
location of the fuel vent itself. There are specs for the location of
these if you look in the manuals. But for the fact that the fuel doesn't
migrate when parked, I would have said this is definitely your problem
For instance, Cessnas with bent or incorrectly positioned vents are
chronically out of balance.


Bertie

"Bertie the Bunyip" wrote in message
...

OK, talked to my guy. He had several suggestions. One is to drain it all
dry and then blow some air (carefully!) into the right tank and see if
you can hear it coming across into the left tank by listening at the
filler cap.
Possible problems he came up with were a crushed or swollen fuel line
downstream of where you tested previously, or, he thought more likely,
incorrect placement of the little plastic fuel vent deflector and/or the
location of the fuel vent itself. There are specs for the location of
these if you look in the manuals. But for the fact that the fuel doesn't
migrate when parked, I would have said this is definitely your problem
For instance, Cessnas with bent or incorrectly positioned vents are
chronically out of balance.


Bertie

Oh yeah, that's a lot of help. Where did you find this wanna boi, Wikipedia?

"Maxwell" luv2^fly99@cox.^net wrote in
:


"Bertie the Bunyip" wrote in message
...

OK, talked to my guy. He had several suggestions. One is to drain it
all dry and then blow some air (carefully!) into the right tank and
see if you can hear it coming across into the left tank by listening
at the filler cap.
Possible problems he came up with were a crushed or swollen fuel line
downstream of where you tested previously, or, he thought more
likely, incorrect placement of the little plastic fuel vent deflector
and/or the location of the fuel vent itself. There are specs for the
location of these if you look in the manuals. But for the fact that
the fuel doesn't migrate when parked, I would have said this is
definitely your problem For instance, Cessnas with bent or
incorrectly positioned vents are chronically out of balance.


Bertie

Oh yeah, that's a lot of help.


I know.


Where did you find this wanna boi,
Wikipedia?




Said where i found it wannabe boi.


Aren't we the little prankster!


Bertie

On May 8, 7:39*pm, Bertie the Bunyip wrote:

another useful clue

The deflector business is something nobody else has mentioned yet;
I'll pass it on to the A&P. He's got a couple planes needing
attention to their engines and the Citabria, being under-utilized and
still flyable in the pattern, isn't a high priority, so resolving this
may take until next week. I'll post what's been found after the whole
system's been gone through thoroughly. Thanks.

quietguy writes:



The deflector business is something nobody else has mentioned yet;
I'll pass it on to the A&P.

I can't see the deflector being it, considering it does not drain back
while tied down...

--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

On May 8, 8:18 pm, quietguy wrote:

The deflector business is something nobody else has mentioned yet;
I'll pass it on to the A&P. He's got a couple planes needing
attention to their engines and the Citabria, being under-utilized and
still flyable in the pattern, isn't a high priority, so resolving this
may take until next week. I'll post what's been found after the whole
system's been gone through thoroughly. Thanks.

The deflector does nothing more than keep bugs and ice out of the
vent. Cessna places theirs behind the lift strut to do the same thing.
When the airplane isn't in flight it makes no difference whatever, and
older Champs and Citabrias didn't have it.
I would still suspect rotten hoses, on both the fuel and vent
lines. Especially if they're old or have been run on Mogas. Or if some
mechanic installed the wrong MIL-Spec hose the last time they were
replaced. It's the only thing short of a blocked aluminum fuel line
that would do this, especially if it won't crossflow overnight to the
lower wing or equalize between tanks if the airplane is level. It
doesn't take much slope to make it run from one side to the other,
BTW.


Dan