Citabria Fuel-Flow Problem

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