If you've swapped carbs several times and obtained fair results one time but poor results another time with the same carb I don't believe the carb or the timing is the problem. Timing does not have to be dead nuts on the money for an engine to run acceptably. It can be off a couple of degrees and most people would never note the difference. It sounds more like you have a fuel delivery problem. It's not making the trip from the tank to the carb correctly. That's where I would be looking for air leaks.

Back to the Zama carb. Going with a larger carb will not always provide an increase in performance. Quite often you will experience a loss in performance. If the engine is a conversion then it likely had a carb that worked quite well in the original configuration. Many conversion engines have the same carb they started with since the conversion process did nothing or very little to internal components.

Go back through the fuel lines and see if someplace isn't sucking air.