This week I will discuss the advantages and disadvantages of two and four strokes.  A four stroke has four strokes; the first is the intake a valve open and allows fuel to come into the cylinder. The next is the compression stroke the piston drops and the both valves close, then the ignition stroke the piston starts to move back up and (if it is a gas engine)  the spark plug ignites the fuel after is the exhaust stroke the exhaust valve opens allowing the gas that is made to flow out.  In a four stroke engine the oil is put in the crank case were the moving parts are, so the oil and gas are separate.  In a two stroke engine the gas and oil is mixed and only has one valve the intake valve.  The gas and oil are mixed in a two stroke the gas first flows through the crankcase to lubricate the moving parts then past the piston into the combustion chamber. Then the exhaust freely flows out; this is gives a two stroke more zip. Since two strokes don’t have oil in the crankcase they don’t have oil in the crankcase and don’t have the extra vales and springs and ect. In the snowmobiling world lighter is better, the two stroke doesn’t have as much runtime as a four stroke but they have the quick response and are cheap to rebuild. A four stroke is heaver but has more runtime before a rebuild. But the rebuild is about twice as much as a two stroke.  The newer four strokes are getting to have a quicker response time all the time and are way more efficient.

http://www.animatedengines.com/twostroke.html
The link above shows a two stroke in motion.