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All my carb fiddling experience is with motorcycle side-draft constant-velocity carbs; you can do things with those that can't be done with traditional automotive fixed-venturi carbs.
That engine layout with a single carb, no matter what kind it is, is going to have one troublesome issue: cylinder-to-cylinder variation. The cylinders closest to the carb will be getting something different from the cylinders at each end.
Is that engine a side-valve (flat-head)? If so, they're inherently less efficient than overhead valve engines.
Don't forget ignition timing, either.
Don't hesitate to put modern instrumentation on your old-school engine. A vacuum gauge and an air/fuel ratio gauge might shed some light on what's going on.
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