I've got a YS 120 that doesn't have a lot of time on it and has been the best running engine I have owned. I have always run it a little rich so that it would stay well lubricated. The other day it decided to give me some problems. I took off and after getting airborne it started to sputter and then it died. I landed "safely". I have tried to tune it but the problems I am having are that when I remove the glow driver it starts to act up. When running without the glow driver attached at high throttle it just dies for no apparent reason. When it does this the prop comes loose. Since it appears to run fine while the glow driver is attached then I would assume that it is running too rich but I have adjusted the high speed needle valve throughout the spectrum. It appears to run best when the needle valve is about half of turn out from all the way in which means I am having to run it lean which sounds like it is getting too much fuel. I have replaced the glow plug twice with no luck. Do you think there may be a problem with the fuel regulator? Better yet, what might be causing this problem? Any help would be greatly appreciated.