On a YS four stroke engine, the carburettor feeds the fuel/air mixture into the crankcase and the bottom of the piston pumps the fuel/air mixture into a pressure chamber that leads to the intake port in the engine's head. This supercharges the engine. The supercharging happens because the bottom of the piston pumps air on every downstroke but the intake valve is only open on every other down stroke. Supercharged four stroke engines are painfully loud without mufflers.

If you want the low slime factor of gasoline power but don't want the expense of planes big enough for 50cc engines, consider the zero mess power of electric. I just built a plane from plans that calls for a .60 two stroke/.90 four stroke but I am running a e-flite Power 60 outrunner powered by 4500 mAh 6-s lipo battery. To say that I am not disappointed by this setup's performance would be a huge understatement. I don't even have the the motor propped to max out the amp rating and the plane positively smokes. (not literally, I hope) Those batterys do cost $280 bucks each though but I'll never have to take paper towels and Windex to the field again.