as long as you tune your inverted 2st engine correctly, not overly rich, they run great the biggest mistake is running to rich and having the fuel tank positioned incorrectly making sure the fuel lines line up with the carb nipple, and not to high, so the fuel siphons out of the carb.
the biggest mistake made is flipping a good running engine over and expecting it to run well without re-tuning seems like some 2st engines run better upside down better than others .
as for 4st engines all mine are inverted and they work great