it sure sounds like the rear carb is not flowing fuel correctly, ID check the float levels and verify the carbs feed line is ok, in fact if the carbs are basically identical, ID swap the two to see if the issue follows the carb or stays the way it is, to verify its that rear carb or the fuel line supply issue.
Id sure look for vacuum leaks and verify the ignition firing order, and fuel pressure at each carb inlet!
damn, I wish you were local, so I could drop over and get you sorted out!
that sounds like something rather easy to diagnose and correctly tune,, GOOD LUCK!
there are several aftermarket intake designs some have a single common plenum, some have two separated plenums
yeah, its different in that both carbs need to be synchronized, so both open at identical rates, this is commonly done with a u-tube manometer and vacuum gauge
each carb feeds 3 cylinders on a 6 cylinder engine , and yeah they do frequently have a connecting balance connector tube but thats disconnected/plugged temp for set up.
yeah, old school, multi carb, tune experience helps
