Is the outlet line from the tank within about 3/8" or 1/4" of horizontal to the carb inlet fitting ahen the model is level? If more than that the muffler pressure will have a hard time pushing the fuel into the carb when inverted - causing a lean run.

IMHE engines don't care what position they are in. I run them upright, tipped 90º and upside down. The problem is in the fuel system 80% of the time - either air intrusion or fuel delivery.