Hey it was Crassius' idea not mine!
I still think you should switch the shifters to the opposite side but meh, whatever makes you happy, you know.
My guess, and it is just a guess, is that the fuel is foaming inside the carb bowl because it is not rigidly attached yet. If it has any wiggle room it can shake like one of those paint mixer things. That's why I was saying about putting something between the carb and the frame so they can be strapped snugly together with a cable tie. The foamed fuel would cause a lean a/f ratio, consistent with the symptoms you describe.