One reason I don't recommend used engines is you really don't have a clue as to their history.
The regulator adjustment DOES NOT control the idle. Like all other YS engines it is used to adjust the transition from low to high.
The NC had a fixed airbleed for the idle. Some needed to have this drilled out to get the idle lean enough. Other worked fine as supplied. Obviously this one has the rich idle problem and needs to be fixed.
If you elect to buy the engine and then have it serviced, it could need $100 plus to get it back up to good working order. Or, you could get lucky and it just needs some TLC. No way for me to know without seeing it.

Hope this helps,
Dave