SCOTT42,

This a really good tutorial on tuning the YS 120 and just about all the other YS engines

http://www.geistware.com/rcmodeling/index.htm

Just scroll down to the YS section and click on YS Engines, this gives best RPMs and idle speed.
There are also suggestions on prop selection.

Remember that you want to tune for the RPM that the engine performs "on the cam" where you are getting the best
efficiency and not the maximum RPM.
Usually this is about 8000 RPM, so you want to prop the engine so that it turns at this speed running
a bit on the rich setting, you should set the idle below 2000 RPM.


Good luck


Happy flying, Oscar