I've got planes with the two strokes upside down, right side up, horizontal and four strokes the same way. You have to be a little better at the initial start up so as not to flood them (It's easier to do because the excess fuel will puddle in the top of the combustion chamber and not be pushed out with the exhaust) but they run fine in any position. Think about flying inverted, it wouldn't be so good if they quit running upside down. Just my two