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I have a 72 Elcamino with 4 barrel Q-jet carb. It idles at 750 in park with pcv connected to pcv port at the base of carb. When I disconnect the pcv hose from carb and hook up vacuum gauge the engine rpm drops to about 630. I then re adjust the idle back up to 750 and adjust fuel mixture. I then disconnect the vacuum gauge and reconnect pcv hose and the idle (rpm) shoots up 900+. Can someone explain why?
 

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The reason the idle changes by disconnecting the PCV valve is because it acts as an internal vacuum leak and it will slightly lean out your idle and raise it some just by a hair. On a bone stock motor its more noticeable vs a performance motor with a lot less vacuum. Because you unhooked it and readjusted your idle mixture screws and then when rehooking it up again you have changed your idle setting and once you hook it backup you now have added the internal vacuum leak back to it and thus your idle will be up more.
 

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Correct. A PCV is a metered vacuum leak. I had the same situation in the 350 in my boat. Previous owner put fancy valve covers on that didn't have a PCV hole. It idled very low and rich. If I take the oil cap off, it ran fine.

I would find a different port to test your vacuum so you're doing it "live" with the PCV active.
 
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