I believe you are talking about the valve that connect to the manifold, right below the carb. This should be a pcv valve with some brass fitting , and it may or may not be connected to a swivel valve and then into the 3/16 hard tubing if i'm right. This is for the fording system and since you said if you shut in, believe the shut off valve should be still in the connection line. First you have to remove the whole assembly there, just unscrew from the hard line and then the manifold. When on a bench, use heat as needed and remove the brass fitting (don't damage them). Once down to the round pcv, you can unscrew the top, if you wire wheel it, you will find the marking and arrows to tell which way to unscrew and arrow for installation.
The pcv valve will block about 98 % of the air, but that for back flow purpose to keep water and what not from getting in, you brass swivel valve is what truly cuts or enables the flow. If you are having vacuum issues, i would not really be looking at the pcv, check to be sure, but remember engine suppose to be able run just fine (lower rpm) with the valve closed. I would suggest you mainly check pistons, and of course check for vacuum leaks on the carb its self. Also remember to check vaccum, the engine needs to be up to running temp, or any vac gauge reading will be off. Also the crank speed is a bit lower than running, so that is not the correct way to check. Once again check your pistons for cranking vac.
