FWIW, I have had exactly the same problem as all others on this forum re the YS-63S. Brand new engine purchased about a month ago. Fuel dripping, too high idle, bad transition, dying after throttle down, etc. Last weekend, after many many tanks of rich settings, I started really paying attention to the engine - Top end is now running 11,500+. Low-end needle is open about three turns. I closed it fully clockwise first and then opened it about 3 turns. YS recommends one turn out.

I then started playing with the regulator and lo and behold, I now have a low end idle of about 2,800, a high end of about 11,500+ and a very smooth transition from low to hi. I also have an enormous amount of power. The engine is mounted on a CAP 232 World Models Aubuchon model. I have nearly infinite vertical now! Oh, also, no fuel dripping any more either from the carb. My reg setting is such that the reg screw is slightly above the reg housing. High end needle is about two fully open turns.

My guess is the regulator material and the spring need to "break in." Also, I think this engine may be way to touchy re needle and reg settings.

I have quite a few YS engines, both 4 and 2 stroke. This one was a bear to set. I am hoping that it will keep and that it won't revert back to its previous state of malfunction.

I run a 4 stroke YS plug and Magnum 30% or Coolpower 30% heli fuels. I am really a heli guy at heart but playing around with planks a bit these days.