Hello,

Our 120Fz is about 5 months old with no more than 75 flight on it. it was broke in per-book with wildcat 20/20 fuel running a 16x8 APC prop turning 9100RPMs on top end. Problem is that the engine after about half why thru the flight will just start to go rich, or it seems anyway. It start changing rpm all of a sudden back a forth but it will go away when you bring it to about half throttle. The engine is the best runnig ys i've ever seen in my life, with the low end runnnig at 19-2000 rpms forever inverted with no on-board glow. Its on a H9 Talyor Craft so its not doing 3d aerobatics either. Its not the plug, no trash in the regulator, ou tell me. It blew a head gasket about 20 flights ago, all gaskets replaced with central hobbies gaskets, no air leaks, perfect pressure.

It is possible that this engine does not have a gas ported piston. Without it, the symptoms you describe are quite possible.

Your best bet is to send the engine so that service can verify if it has the piston mod.

Here is the address:

YS Service
1295 Industrial Court
Gardnerville, NV 89410

Phone 775 782 4562

Sorry for the hassle, but it will be easier in the long haul to let us see what the problem is.

Dave