Sounds to me like butterflies are not closing completely all the time,
1—take the tank and pull the throttle cable out, check if it slides easily, if not clean and lube
2—while cable is off the carbs, does the butterfly shaft snap back when loaded/opened, if not a light spray on the springs of each carb, {may be full of binding gunk} then clean them with a spray of fuel and respray with lube,
That’s the simples,,,,, it may be leaning out
1– spray some high flash point cleaner {aero start or carb cleaner or such} around the intake rubbers between the carbs and motor {rear bells have no bearing on this},
—-it will either rev up if a slight leak, or die if a large enough leak is present
2—if you still have the octopus fuel intersection check to make sure the vacuum hose is not cracked,{will lean out #2}
3—do you have paper gaskets between the intakes and the head, {the paper fails over time}
4– with the carbs out and manifolds removed, check the internals of the manifolds, the cracks can seem quite severe on the out side but not be going through the entire wall, a quick easy fix is a smear of rev silastic over the outside of the manifolds as a stop gap that will/does last for yonks
We always say to change just one thing at as time to eliminate and solve the problem, but with carbs, when they are out it is a damn sight easier to tackle all of this at the same time
