The factory setting for the pump is to set the screw head flush with the housing. Sounds like you may be lean. To richen the pump, it works the opposite of a needle valve, you screw it in to richen. One more thing I just remembered that I have seen on every YS170 is that the throttle barrel tends to stick. This was solved by honing the barrel with some fine grit paper and some lubricant. Note that XMANS pointed out earlier that the ones he has set up end up being about a full turn in on the pump.