I have the YS53, /63, /91, /110 & /120, mostly in pairs. It is a wonderful product, good for an extra inch of prop diameter compared to the OS/Saito type (which I also have & enjoy). They are a little trickier to tune than a standard 4-stroke, but not a lot. There is the added variable of the regulator screw, which most of the time can be left flush with the crankcase and no special trouble. The pumped, regulated, pressurized tank simplfies tank plumbing, and I prefer this kind of fuel delivery system over any other.

Four strokes all come with instructions saying you need to set the valve clearances every so often. In my experience, once a season is usually enough, but I did have a (non-YS) 4-stroke once that would not hold it settings well.

The high nitro is a disincentive for some. If you burn just a gallon or two a year, then the added cost is not so bad. If you fly hard several times a week (like us retired old farts), you can buy high nitro in bulk and it costs what sport blend costs at LHS retail. If you're in the middle, buying 3-6 gallons a season, then you may resent the added cost. The YS is not happy if you skimp on nitro or lube--gotta be 20/20 on up to 30% nitro if you like (and I do).

The YS has a wonderful supporter in Troy Newman, whose tuning posts are legendary. Look them up, and you'll know everything there is to know about tuning a YS.

HOWEVER, with specific regard to the 110: I bought a new YS110 last year, broke it in according to the book. Tuned perfectly on the ground, idled @ 2000 rpm, throttled up beautifully, fuel Magnum I, APC 15x8, everything right on the money. However, after the first ten flights it started hesitating when I would throttle up from flying idle to cruise power--as in coming down from a stall turn, for example--it would seem to gag for a moment, then catch and go. Pretty soon it would gag and die. For the next few weeks, it kept doing this, and I kept trying to tune this out. Just about every flight ended in a deadstick after a few minutes flying.

I posted for help to the YS engine forum, got a response from Dave Shadel, whose answers are usually clear and effective. No improvement. Then not too long ago I saw a post from Seanychen here on RCU describing the same problem, citing many other 110's with the same problem, blaming air leaks (all with new engines!). Seanychen's contribution was to turn the regulator all the way in, then back out three turns, problem went away. I switched to another YS110 last year (bought used, flew great), and have not yet had an opportunity to try the three-turns solution.

If you are strictly a plug and play guy, get yourself an Evo from H9 and have yourself a good time. If you are not afraid to twiddle with an engine, the YS is definitely worth learning how to tune and maintain. It helps tremendously to have a YS guru locally who can help you on occasion, but I was able to learn it myself with the help of the Troy Newman posts. Power to weight the YS is in a class by itself.