Look.
YS engines are the most powerful 4str. available.
But,
tuning is a thing to think about it.
My friend has many YS engines.
He builds onlly kits,not even one ARF trainer.
He says that he trust YS because they don't quit and also they pressurize the tank....=>constant fuel delivery to the carb.
So,until now i've seen three of his plains crashing hard.
A Marutaka Corsair with a YS 63 FZ-S crashed first flight.
Not a construction problem not fuel.
He uses Cool power 15% nitro.
Second was [X(] Cap 232 from Tettra.
It had many flights with it,no problem or messing with the YS 91 AC that has.
But one and a half month ago we went on the field,he took off and after the first turn the engine just quitted.
...maybe not good reflection characteristics + windy day and crashed it.
And the third was an Acrostar Tettra kit with a YS 110 FZ.
And things went exactly as with the Cap above.
I am not trying to discourage you.
Me?
I bought a YS 53 FZ used,it was in a very good condition.
Good bearings etc.
But couldn't tune it to run correct except idle.
Maybe it was an old lemon ofcourse.
So,
back again to the topic,
needles it all engines must be inspected ones in a while.
You don't tune an engine and leave it.
You check it periodically.
And when we are checking it's power?
The best.
It is the most powerful engine in it's class in comparison with all other 4strokers.
The only,
messing things i dislike in a YS are:

1) High nitro percentage=>that costs a lot.
2) Hard to tune it because it is NOT a SIMPLE engine.

I'd better go with an OS 120 FS Surpass pumped if weight is not a problem.
The YS 110 FZ weights 800gr with muffler vs the OS FS 120 Surpass/p that weighs 1000gr.
For 200gr i would go with the OS.
And i would be sure about what i bought.
Enjoy.
Kostas
Greece