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?