Hmm! I just took a look at my YS 63 and sure 'nuff one of the o-rings has pushed out of place in the head also. This is a brand new engine. I reported above that it ran at 4800 rpm with the throttle completely closed...indicating a big air leak somewhere I figured.

I pulled the throttle barrel out of the carb and found the barrel seal torn in to. However I may or may not have done that pulling the barrel out, no way to know.

New seal on it's way and I have reset the o-ring on the pushrod tube. I don't know if it can cause enough air leak to cause my problem or not??
If it did, I'm a little disappointed in having that kind of problem in a brand new engine and my first experience with a YS.