Check the float levels. On my friends 6-pack we could not get the end carbs float level down. He has an electric fuel pump and was putting out 9-PSI which was bending the float arms on the outer carbs.
The outer carbs used the nitrofil float and has aluminum float arms, the center carb used a brass float with the brass float arm and the extra pressure did not seem to mess up that center carb.
Long story short, reduced fuel pressure to 5-PSI, straightened the bent float arms, and readjusted the float levels.