I'm having a tremendous problem with my YS 91. I only have about4 flights with this engine and is killing me!
here are my problems or symptoms:
1) At iddle the engine have a change in rpm (about 100 to 200).
in about a minute after it accelerates and desacelrates, the engine suddenly quits. (i changed the spar several times)im usng an OS plug!!!

2) At full throttle the engine is only giving me about7,000 to 7,500 rpm with a APC 13 x 11. (i think this is too little rpms)

3) In one of the flights I inverted the plane and the engine lows in rpms and then quits!!

other considerations: im using Cool Power 15% nitro!
I dont know if this is good, but my high speed neddle valve is about 1 1/16 open... (is that too much closed?, but thats when the rpms are at it highest)

thanks for your answers....
Alex