2-stroke engines are not very common because of their low efficiency, which is caused by a bad cylinder capacity. A 2-stroke engine emits many polluting exhaust gases, because not only gasoline, but also oil is combusted. A blue plume of smoke is created in the exhaust when this mixture is combusted.
 
2-stroke engines have higher engine speeds, which causes them to wear out faster. Once every two strokes, the crankshaft rotates and creates a high power. The spark plug is worn out easier, which causes the engine to start more difficultly or even not at all.
 
These engines are built more lightly and simply. That is why they are cheaper than 4-stroke engines.
 
Disadvantages of a 2-stroke generating set:

• low efficiency because of bad cylinder capacity
• polluting exhaust gases (too high emissions) + oil combusted as well
• noisy
• mixture gasoline and oil
• more wear because of high engine speed
• difficult start

Advantages of a 2-stroke generating set:

• no valve operation
• easy lubrication
• cheap engine

Extra link: http://en.wikipedia.org/wiki/Two-stroke_engine