[Flathead Fever]

The PCV valve is extremely important for preventing your oil from turning into sludge. I'm not a big fan of how they look on a vintage flathead but they will absolutely increase the lifespan of the engine.

The air fuel ratio will change some with the installation of a PCV valve. Its going to lean it out a little. You will need to adjust your idle A/F mixture screws and your idle speed screw to compensate for the controlled vacuum leak you just installed. At idle the PCV valve is almost closed, this is when Intake manifold vacuum is high. The vacuum closes the valve to its smallest opening so it does not lean out the mixture that much at idle.
As the speed increases the vacuum drops off allowing a spring to open the valve further, when the blow-by is at its greatest and the engine can handle burning the excessive blow-by.

The only thing that goes wrong with a PCV valve is it can get stuck in the wide open position which will allow too much air to flow at idle. The engine will die or run extremely rough at idle. If you pinch off the hose to the PCV valve or stick your finger over the end of the valve and the idle straightens right out you have a bad PCV valve. It should rattle when you shake it. Also over time the spring weakens and the valve looses its calibration allowing too much air flow at lower operating speeds. They are a maintenance item that need to be replaced probably every 50K miles on a daily driver.

Choose a valve from an engine that is close to the same cubic inch as your flathead. It will be designed to flow the correct amount of air for that cubic inch s without leaning out the engine too much

On an overhead V8 like a 302 Ford the PCV valve plugs into a rubber grommet in the valve cover. The valve is connected by a hose that is compatible with fuel and oil, Heater hose will swell up and turn soft. The vacuum will then be able to collapse the hose. Use fuel or emission hose only. The vacuum source hose is plumbed directly beneath the carburetor so it feeds all the cylinders evenly and does not lean out any one cylinder.

Look at the illustration on post #24, See how its designed to pull air from one side of the engine to the other the other-side to purge the crankcase. You want to try and create a flow like this inside your flathead. You do not want to mount the fresh air inlet too close to the PCV valve otherwise the PCV valve will just be sucking in the incoming air and not the crankcase fumes.

There are open PCV systems and closed PCV systems. The difference being if the oil breather has a hose connected to the air filter. The early open systems allowed excess blow-by to go out the breather cap and into the atmosphere. That was creating smog so they closed the system. The closed system routed any excessive blow-by that the PCV valve could not handle out the oil breather cap, through a hose and into the air cleaner where it gets sucked into the engine and burned. One of the most common failed items on a smog test is somebody replacing the closed type oil breather cap with a chromed open style one that allows blow-by to escape to the atmosphere. You need to make sure you install a closed style breather cap.

In order for the PCV valve to suck air out of the crankcase there needs to be fresh air allowed to flow into crankcase. This filtered air flows from the air filter on the opposite side of the engine, through the breather cap and into the crankcase. Like I said before, its important that the fresh air going into the crankcase is at one end or one side of the engine and the PCV valve is on the opposite side so there is a nice continuous flow going through the crankcase. If its working good you can pull the breather and lay a sheet of paper over the tube and it will suck the paper down.

They make a couple of difference hose connections to convert your air cleaner into a PCV system. Mr Gasket makes one that you drill a hole in the air cleaner base and bolt on the hose connection. You need a good sized air cleaner so the drilled hole will be after the air has gone through the filter. If you have small hot rod air cleaners K&N makes PCV breathers you could adapt to the push on oil cap. This way you could eliminate that filtered air hose from the air cleaner.