Two-stroke engines often provide higher power-to-weight ratio, usually in a narrow range of rotational speeds called the power band, compared to 4-stroke engines, have a greatly reduced number of moving parts, are more compact and significantly lighter.
The first commercial two-stroke engine involving in-cylinder compression is attributed to Scottish engineer Dugald Clerk, who in 1881 patented his design, his engine having a separate charging cylinder. The crankcase scavenged engine, employing the area below the piston as a charging pump, is generally credited to Englishman Joseph Day.
Gasoline (spark ignition) versions are particularly useful in lightweight (portable) applications such as chainsaws and small, lightweight and racing motorcycles, and the concept is also used in diesel compression ignition engines in large and weight insensitive applications, such as ships, locomotives and electricity generation. The heat transfer from the engine to the cooling system is less in a two-stroke engine than in a traditional four-stroke, a fact that adds to the overall engine efficiency; however, traditional 2-strokes had a poor exhaust emissions feature.
The two-stroke petrol engine was very popular throughout the 19th-20th century in motorcycles and small-engine devices, such as chainsaws, out-bored motors, and was also used in some cars, a few tractors and many ships. Part of their appeal was their simple design and low cost to manufacture and often high power to weight ratio. The lower cost to rebuild and maintain made the two stroke engine incredibly popular, until for the USA their EPA mandated more stringent emission controls in 1978 taking effect in 1980 and in 2004 taking effect in 2005 and 2010. The industry largely responded by switching to four-stroke petrol engines, which emit less pollution. Most small designs use petriol lubrication, with the oil being burned in the combustion chamber, causing discolored blueish smoke and other types of exhaust pollution. This is a major reason why two-stroke engines were replaced by four-stroke engines in many application. So two strokes have been around for a long time and are used in many things i think they should have not stopped making them because they were really awesome little motors. were i got this information http://en.wikipedia.org/wiki/Two-stroke_engine