Your experimenting has pretty much proved what some of us suspected from the beginning. The carburetor in question does not supply sufficient air nor fuel to get the best performance from the engine, nor does it meter the fuel properly across the range of operation from idle to power production to light throttle cruise. The carburetor provides barely adequate performance- - - - -not optimum. It's better than a worn out, dirty carb, but not by much. Over time, it will probably deliver sub-part fuel economy as well. You might- - - -but probably won't be able to improve things by installing a bigger venturi and opening up the main metering system to flow more fuel than the wide open main jet can supply. Doing that properly will require more equipment and a more thorough understanding of carburetor theory and modification than you currently have at your disposal.
Jerry
