I have been looking and may I missed it. but is there a site or link that would explain the differance in YS engines. Such the 120NC / FZ/ SC. a friend is selling a new 120NC, and I just want to know the about the types of motors.. thanks Terry Green

There is no site or link. The info has been recounted many times on RCU. Suffice to say that the NC was the most troublesome as it did not have an adjustable idle, only a fixed airbleed which may or may not work for your local conditions. The only fix was re-drilling the hole to a bigger size to lean the idle, a real pain in my opinion.

The FS was the first and most reliable 120 , the SC made monster HP, but was a very limited edition, and the FZ was the last and probably the best from an operational and power standpoint. Then YS dropped it and went to the 140 Sport, which is a bored and stroked 120 in the same case.