My 3W-100 suffered with this problem going rich when inverted. I rebuilt the carb - new gaskets, diaphragms and inlet needle. That didn't do much. I put in new NGK CM6 spark plugs. That didn't do much. I installed the 90 degree carbon fiber intake horn and reset the needles on the carb. That fixed it.

If the engine is running rich when upright and level, the excess fuel will pool in the bottom of the reed cage. If you roll inverted, this fuel will get dumped onto the spinning crank and get whipped into the transfer ports, making the engine burble very rich. The intake horn helps to ensure that the mixture is correct over the entire throttle range so fuel won't puddle.