340 fuel/ignition issue?

[1] Make sure the base gasket seals all the passages in the carb base. A leak here causes strange things.
[2] Make sure the choke blade is vertical, not flapping around, engine running.
[3] Make sure the sec air valve dashpot is working; should pull in with engine idling. With engine idling, make sure the connecting rod from dashpot to AV moves freely.
[4] Engine off. Push in dashpot plunger & release it. Compare to the other carb. If it is slow, the diaphragm has gone hard, causes a rich mixture.
[5] Make sure the AV moves freely. Should be at 2 turns as a starting point.

Keep us posted!