Good point about who put the parts in the engine and the cam timing.

One other thought, before we go any further, have you checked the valve clearence?

Alberto, considering a YS can be temperamental due to is high performance and not knowing the history on the repair of the engine, you might want to consider sending it to Richard at YS. You said you are new to 4 strokes and the YS is not like other 4 strokes so making sure you are starting with a good baseline to learn with might help a lot more than chasing symptoms. Once you get the YS figured out they are a great engine, but starting with a problem child makes a lot of people shy away from them.