I would be inclined to think the tank would be too high if anything. An inverted engine should be able to run,idle, and accelerate just as well as a side mount or upright engine IF you get the tank at the right level and get the mixtures ( high and low speed) properly adjusted. I suggest you run the engine and get the mixtures properly set with the airplane upside down (engine upright). This may or may not be possible if the tank is way high or way low . assuming the level of the tank is more or less at the right level and you can the engine to run and throttle properly then you know the engine and the carb are not the issue. Then it is almost certainly a tank issue.