They aren't really screws, at least if we are talking about HS6's, which is what a PV would have come from the factory with. Rather a large nut that goes around the bottom end of the jet. (In the picture, at the bottom of the main carb body, about 1/4 inch above the red base of the jet - not all jets are red like that though!)

Screwing it 'in' (facing the outside of the carb, moving the outside edge of the nut forward) raises the jet, and pushes it farther onto the tapered needles, which leans the mixture. Screwing it 'out' lowers the jet, and richens the mixture. It's sort of confusing to talk about clockwise or counter clockwise, because you reach up underneath the carb to adjust it, but look down on it from above (unless you are really short!).

There is a whole sequence you use to arrive at the proper mixture, which involves using a little pin located surreptitiously under the edge of the dome to raise the piston slightly, and noting what the idle speed does when you do that. Google up an article or two, but here's the gist about how to read what happens when you use the pin to raise the piston:
RPM's rise and stay up, that carb is rich.
RPM's rise briefly, then drop, mix is about right.
RPM's fall, engine gets rougher - mix is lean.
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I'm JohnMc, and I approved this message.