If you're talking about why the gas doesn't just fall out the carb it's because there is a pressure differential that causes suction; the fuel is still pulled into the engine even without the help of gravity.

The venturi narrows causing the air to speed up as it passes through and therefore pressure to lower. Also, when the crank opens to let fuel in there is low pressure in the crankcase because the piston is increasing the volume inside when traveling up. The air/fuel in the carb rushes in to attempt to neutralize the pressure. How do you think the fuel gets into the cylinder from the crankcase when right side up? The piston travels down increasing pressure in the crankcase and decreasing pressure in the cylinder by shrinking/enlarging their volumes respectively. When the two areas open up via a port in the sleeve the fuel/air again rushes up into the lower pressure area, despite the best attempts of gravity.