I found some great directions for tuning YS engines. When in doubt, this is what I resort to (requires a tachometer):

At WOT, set high needle for max RPM, then back off a few hundred RPM with high speed needle.

Next, drop throttle so engine is at EXACTLY 4000 RPM, and watch what happens for the next 10-15 seconds. If RPM increases, regulator is lean and needs to be made rich. If RPM decreases, regulator is too rich and needs to be leaned.

Last, drop throttle to 2000 RMP and hold, see what happens over next 10-15 seconds. If RPM increase, low mixture is lean, needs to be made rich. If RPM decrease, low end is rich and needs to be leaned.

I'm pretty new at this YS thing but sounds to me like some things are rich on your engine. Best to check everything as it is a 3 part system and all need to be working together properly.