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.