Once it is running it doesn't care what position it is in. Getting a good prime is more of a problem when sideways or inverted though. They need to be internally wet to fire and a couple of flips with a finger over the carb intake or muffler exit doesn't do the trick because the fuel drops just run out of the carb and onto the ground. So when cold, without the glow ignitier on yet, 1/8 throttle or so, and covering the intake to choke it, spin the motor for about 20 seconds with the electric starter. This will actually suck some fuel into the engine. Then apply the glow power and it should fire.