Dave,
Other than checking the valves, what kind of routine maintenance should be done on YS engines to provide a long life?

I ask this because my YS 120FZ broke something internally after about 220 flights in a Boxer 120 pattern plane. There's now a bump and crack visible on the bottom of the crankcase, so I'm guessing the engine is toast and not worth repairing (it's 1-1/2 years old and no longer under warranty).

Anyway, I've now got a new YS 140S (sport) to replace it and also a new 140L in my other pattern plane.

These are expensive engines and I want to take good care of them. I don't want a repeat of the 120 experience. If this was just a freak thing, that's fine -- I just want to make sure I'm not making some kind of operator mistake.

Should these engines be serviced annually? If so, what should be done and who should do it?

I'm running Wildcat YS 20/20 fuel with OS F plugs. I tend to run on the rich side and notice a fair amount of carbon on the valves. Could that cause anything catastrophic in the long run?

Are there any symptoms that I should keep an eye out for as warning signs of impending trouble?

Thanks for any advice you can offer.

I think that leak free gaskets would be nice for lower maintanence? Would I be right?

You really need to send this back to us for a look. We may be able to help you out on the repair.
There is not much you can do to prevent failures other than periodic replacement of part like rods and bearings. Most guys don't do it and some never have a failure. Other guys do it and never have a failure. Take your pick.
My personal feeling is that the big YS engines need new rods and bearings every 100 flights, but the lack of failures does not support my thinking. It's what I would do. I know of YS 140 engines with 300 plus flights that have never been apart.

When you say rods, do you mean the pushrods or the connecting rod?

Conrods.