Its always best to mount them anyway but inverted,but yet thats probably how most 4-stroke are mounted and properly set up,they'll run great! Sideways work good,and right side up is always best. If you must mount inverted,simply make sure your tank height is very close to where it should be......make your fuel tubing from your tank to your carb have a small arc in it where as the peak of the arc is higher than both your tank and your carb so it cant just drain right out. Just make sure you dont get the "siphon" effect going,or it will still drain out. And probably most important of all(and how most people do without ever realizing it).....when your kill your engine after flying or whatever...kill it by trimming your throttle bar beyond where it can run and keep it there until after you fuel up and ready to start your engine. This usually blocks the carb enough so that the fuel cant get to the carb while your fueling up.

And I'm assuming your fill line isnt "T'ed" into you carb line.....if so,dont do that either because that always a bad idea.