I have had inverted engines so low the fuel would run through the carb. I would clamp the fuel line with a hemostat prime the engine, start it and take the hemistat off. I had to turn the needle down a bit and the idle was a bit tricky, but as long as I did not fly inverted it ran fine. I have also seen tanks set real low, as long as you have muffler pressure,especially at idle you can get it to run. In fact one poster here has had tanks mounted about 6 inch's below the engine on an air boat and he was able to get it to run, though Ithought it would be an issue that low.