Well, the site blew up my first long reply, so will try again.
Sounds like you've fixed so things that need fixing, but have you considered a possible air leak around the shaft seal of the fuel tank selector? (I went round and round with symptoms similar to yours before this resolution.)
A test is fairly simple ground test is to tee into the output of the gascolator line running to the engine-mounted fuel pump. Put the line into an open container and turn on the fuel pump. After a few seconds, fuel should flow smoothly into the container, without spurting. Hold the line into the collected fuel; there should be none, or minimal, bubbles. If the shaft seal o-ring has failed, the pump will happily suck in air as well as fuel - it pumps both equally well. Never mentioned anywhere is that, especially with low wing airplanes, this o-ring is a wear item.
The spurts will drive the carbs crazy and ironically, causes the engine to run rich. How? Splurts splash fuel out the vents which are first led to the top of the intake log and get sucked right back into the engine. After fixing this o-ring, my fuel consumption dropped by as much as 1 gph, from about 5 to about 4 gph.
