what prevents unmetered air through PCV??

The vacuum in the manifold is greatest at idle, as Scoutpilot says. In this case, the plunger is pulled against the spring inside the PCV, and the plunger cone seats against the end of the valve case. In this case, the only port free to admit air into the manifold is the small hole at the end of the plunger. This means at low revs, less air as admitted.

At higher revs, when vacuum is less in the manifold, the spring lifts the plunger cone off its seat. Now, air can be admitted not only through the small hole on the end of the plunger, but also through the other larger holes in the side of the plunger, and around the nose of the plunger. ie at higher revs, more air is admitted.

This is how the valve regulates the amount of unmetered air being admitted to the manifold. As Scoutpilot says, this is in a small enough quantity not to upset the metered air/fuel ratio already fixed by the carb. I don't know how much unmetered air can be admitted before this ratio is upset. Only the engine designers would know this. But look at the dia of the throat of the carb and the dia of the PCV plunger holes and we can see that the differences are large. My rough calcs show that the carb throat area is about 760 times larger than the small plunger hole area. So the small plunger hole would admit about 0.1% of the air coming in through the carb throat. The larger plunger holes might increase this 5 times to 0.5%. I haven't measured them accurately, but it would be of this order of magnitude.

Lets assume that the PCV lets in maximum 0.5% of unmetered air and that this is not enough to upset the carb air/fuel mix. Or alternatively, we can assume when fine tuning your carb you can set the mixture screw 0.5% rich to compensate for the addition of this air after the carb. Either way, it is a negligible amount of unmetered air being introduced.

There are two other important things to consider. The first is that the air inside your crankcase is not at atmospheric pressure, it is lower than atmospheric pressure, so it is not the same as "outside" air. The air in your crankcase comes from the oil filler tube which is itself connected to your air crossover tube, all connected to your oil bath air filter. This air is pulled through your oil bath filter, which ony happens due to the vacuum in the engine, and as a result all air after the oil bath filter is sub-atmospheric in pressure. Air flow through the PCV will be proportional to the difference in pressures, so the PCV connected to your properly sealed crankcase (at subatmospherc pressure) will admit less air to your manifold than your PCV exposed to atmospheric "outside" air. That is why it is important that your crankcase is properly sealed, as mentioned on the post by Joe W above.

The other possible factor is that your PCV plunger is not seating properly (admitting too much air at idle) or somehow assembled wrong. These would be easy to verify.

You are excited to get your engine running, and you deserve to be. My advice would be to get it all properly assembled, all parts connected (including filler tube, filler tube cap gasket, crossover tube to filler tube connection, crossover tube to air filter, air filter oil bath filled correctly etc.), then hunt for and eliminate all vacuum leaks (these are really common around the carb base and inlet manifold-to-block gasket), and only then worry about fine tuning the idle on your engine. If you jump straight to trying to fine tune the idle on your engine before these other things are attended to you will spend a lot of time chasing it, and get very frustrated in the process. 

Be patient, take your time, eliminate the things mentioned above. You will get there.