Engine numbers for Yamaha motorcycles, which are stamped into the engine block, are decoded and partially listed on a number of websites. The National Highway Transportation and Safety Administration introduced a standard in 1981 where a motorcycle's engine number had to match its vehicle identification number.
The unique 17-character number encodes information about the vehicle. The first and second digits describe the country and manufacturer, while the third is either an "A" or a "1" to designate it as a motorcycle. The fourth digit describes what kind of motorcycle it is, and the fifth character notes the engine displacement in a range of cubic centimeters. The engine type of the vehicle is coded into the sixth digit.The seventh and eighth characters are used by the manufacturer to describe the design sequence and model version. The accuracy of the identification code is checked with the ninth character, and the 10th through 17th digits are the unique vehicle code, beginning with the year and plant of manufacture.
OK, with that said; Here is a link that may help you decipher your engine.
http://www.xjbikes.com/forums/threads/tech-topic-model-id-and-vins.14577/Hope that helps.
Steve
