I would re-set the valves, hi and lo needles and pressure regulator to factory settings, try out the YS 20/20 when you return to the States (It's going to make a BIG difference, I've never had a YS engine that didn't run nicely right from the start) and try props that put it in the right RPM range. If you bought your engine overseas I'm not sure of the information included with your engine. Tuning instructions from the US importer can be found here...

http://www.yspartsandservice.com/pdf/FZ63S.pdf

Also you can try the YS forum here on RCUniverse. There's a great deal of information about setting up YS engines.