Whirley Bird,
BigBird1 is definitely correct about the YS engines. I fly a YS and it definately can kick back and throw the prop. In fact, kicking back and throwing the prop is usually the best that can happen in a YS. I have seen a YS 120 kick back and tear the engine, motor mount, and firewall right out of the plane (luckily this didn't happen to me). But what causes it???? Running the engine too rich. With a YS engine especially, and most engines in general the richer you run it the more change you have of it kicking back.

Remember that running the engine rich actually means "fuel rich", meaning there is more fuel than air in the engine. That's why an engine running rich "slobbers" out of the muffler, the excess fuel is pouring out of the muffler. And you have to also remember that although it is difficult to do, you can have ignition in the cylinder without a glow driver. Just the compression of the fuel in the cylinder can get ignition of the fuel. What is happening when running rich is that you are getting pre-ignition (the fuel/air is igniting before the cylinder gets to the top of it's stroke) which causes the piston to reverse direction and go back down. viola, the engine immediately reverses it direction and kicks the prop nut off. Now you can see that this happening can cause all kinds of other damage too. The higher end, higher compression engines like the YS are definitely more prone to this because of the higher compression they run. And running 25% fuel just adds the this problem because you have more nitro in the mix when the fuel ignites. More nitro means more bang, and more force reversing the engine. i.e. more force to kick the prop off.

I would like to thank my LHS for this info. The owner of my LHS is Mike McMurtry (a.k.a. father of Sean McMurty, of TOC fame). When I got my first YS motor he told me to NOT run it rich for this very reason. Even in the break in period you don't want to run rich.

So if you are having major problems with your prop getting kicked off, try leaning up your motor a click or two.