Just a little advice. Please take it for what it is worth as I am not a heli engineer by any means, just an average guy learning from mistakes!
When you are mounting your engine to the Vibe 90SG motor mount (I have a YS91SR), the instructions rightfully indicate that there will be a very small amount of "play" in the bolt holes on the engine, so you need to either push the motor up or down all the way in the motor mount before tightening the bolts. With the YS91SR, I pushed the motor all the way up, because I assumed that this would get the maximum amount of the clutch inside the clutch bell.
I had lots of high frequency vibration in the tail of the heli for several flights, and I couldn't get the clutch stack aligned so the starter hub would easily stop rotating the in the one-way when the engine was running. Then, with all of the vibration, the starter hex flew off in flight.
I didn't realize it but the top of the engine bolt was rubbing the bottom of the starter shaft in the one-way bearing, and I think this was causing the issue.
So...to make a long story short, if you pull the motor all the way DOWN, instead of up when tightening the engine mount bolts, this is the way to go. Now, there is much less spinning of the starter shaft when the motor runs and the heli is significantly smoother.