Citabria Fuel-Flow Problem

I can think of a simple test. Tie the aircraft down, stick the
tanks, confirm the asymmetric state.

Now, remove both caps. Wait a while, maybe a few hours. Stick tanks.
Any change? If yes, it's vent related, if not, the [bottom] fuel
lines/manifold.
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The Citabria's fuel system is okay now; those who guessed "filler
caps" have the satisfaction of being right. The A&P had time to give
the system a good look late last week and we tested the fix with some
local and cross-country flying over the weekend. The left tank still
tends to contribute a bit more than the right in flight, but "it's
always done that." And the system now crossflows on the ground when
the wings are not level, which it had stopped doing.

The A&P didn't find any soft or swollen hoses so he took a close look
at the cap gaskets and found that looking like new and performing like
new aren't the same thing. There were no cracks or other obvious
flaws but both gaskets were equally bad at sealing; that's why a
simple cap-swap didn't change anything. It seems that the left tank
just isn't as sensitive to venting/sealing problems as the right. To
seal well, even with good gaskets, the caps have to be difficult to
screw down tight.

Thanks to all who contributed ideas.

On May 19, 5:27 pm, quietguy wrote:
There were no cracks or other obvious
flaws but both gaskets were equally bad at sealing; that's why a
simple cap-swap didn't change anything. It seems that the left tank
just isn't as sensitive to venting/sealing problems as the right. To
seal well, even with good gaskets, the caps have to be difficult to
screw down tight.

The LH tank vents directly outside, while the RH tank vent into
the left, so a leaking RH cap would hold back the flow in flight
somewhat due to the small resistance of airflow through the vent
crossover. But leaking caps still don't explain the lack of crossflow
overnight; only a totally plugged vent system or plugged fuel lines
would do that.
Those cap gaskets compress with age and get hard, and won't seal
unless they're really clamped down tight. New gaskets fix that, but
they can be so tight that the cap doesn't get screwed on all the way
and can come off in flight. I painted red stripes on our caps so that
the stripes are lined up fore-and-aft when the caps are all the way
on. And I use a little tiny bit of Dow DC-4 silicone grease between
the cap and gasket so the cap doesn't have to grind the gasket against
the filler neck; the cap goes on really nice after that.

Dan

wrote in news:6cbe76a7-0b6a-4d7c-a352-
:

On May 19, 5:27 pm, quietguy wrote:
There were no cracks or other obvious
flaws but both gaskets were equally bad at sealing; that's why a
simple cap-swap didn't change anything. It seems that the left tank
just isn't as sensitive to venting/sealing problems as the right. To
seal well, even with good gaskets, the caps have to be difficult to
screw down tight.

The LH tank vents directly outside, while the RH tank vent into
the left,

Not according to my manual, which has a T fitting feeding air to both
tanks from the one vent on all models. It wouldn't be the first time the
reality differed from the manuals, of course.


Bertie

"Bertie the Bunyip" wrote in message
...

Not according to my manual, which has a T fitting feeding air to both
tanks from the one vent on all models. It wouldn't be the first time the
reality differed from the manuals, of course.


Bertie


Or the first time reality differed from your recollection.

"Maxwell" luv2^fly99@cox.^net wrote in news:FIqYj.87799$y05.56401
@newsfe22.lga:


"Bertie the Bunyip" wrote in message
...

Not according to my manual, which has a T fitting feeding air to both
tanks from the one vent on all models. It wouldn't be the first time
the
reality differed from the manuals, of course.


Bertie


Or the first time reality differed from your recollection.


True enouogh, but then I do still remember where I left the manuals, so
it doen't matter.



Bertie

On May 19, 7:00 pm, Bertie the Bunyip wrote:
wrote in news:6cbe76a7-0b6a-4d7c-a352-
:

On May 19, 5:27 pm, quietguy wrote:
There were no cracks or other obvious
flaws but both gaskets were equally bad at sealing; that's why a
simple cap-swap didn't change anything. It seems that the left tank
just isn't as sensitive to venting/sealing problems as the right. To
seal well, even with good gaskets, the caps have to be difficult to
screw down tight.

The LH tank vents directly outside, while the RH tank vent into
the left,

Not according to my manual, which has a T fitting feeding air to both
tanks from the one vent on all models. It wouldn't be the first time the
reality differed from the manuals, of course.

Bertie

Must have changed at some point. Both of ours have the vent
feeding through a check valve into the LH tank, and a crossover vent
connection between the inboard walls of both tanks.

Dan

wrote in message
...
On May 19, 7:00 pm, Bertie the Bunyip wrote:
wrote in news:6cbe76a7-0b6a-4d7c-a352-
:

On May 19, 5:27 pm, quietguy wrote:
There were no cracks or other obvious
flaws but both gaskets were equally bad at sealing; that's why a
simple cap-swap didn't change anything. It seems that the left tank
just isn't as sensitive to venting/sealing problems as the right. To
seal well, even with good gaskets, the caps have to be difficult to
screw down tight.

The LH tank vents directly outside, while the RH tank vent into
the left,

Not according to my manual, which has a T fitting feeding air to both
tanks from the one vent on all models. It wouldn't be the first time the
reality differed from the manuals, of course.

Bertie

Must have changed at some point. Both of ours have the vent
feeding through a check valve into the LH tank, and a crossover vent
connection between the inboard walls of both tanks.

Dan

You're just feeding a troll. All he does is repeat what he is told. Very
much like MX.

In article , Maxwell says...


wrote in message
...
On May 19, 7:00 pm, Bertie the Bunyip wrote:
wrote in news:6cbe76a7-0b6a-4d7c-a352-
:

On May 19, 5:27 pm, quietguy wrote:
There were no cracks or other obvious
flaws but both gaskets were equally bad at sealing; that's why a
simple cap-swap didn't change anything. It seems that the left tank
just isn't as sensitive to venting/sealing problems as the right. To
seal well, even with good gaskets, the caps have to be difficult to
screw down tight.

The LH tank vents directly outside, while the RH tank vent into
the left,

Not according to my manual, which has a T fitting feeding air to both
tanks from the one vent on all models. It wouldn't be the first time the
reality differed from the manuals, of course.

Bertie

Must have changed at some point. Both of ours have the vent
feeding through a check valve into the LH tank, and a crossover vent
connection between the inboard walls of both tanks.

Dan

You're just feeding a troll. All he does is repeat what he is told. Very
much like MX.

Bertie?

A troll?

My gawd, what is the world coming to?


--

"Tis an ill wind that blows no minds"

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