PCV Valve switch

[Charles P.]

I just changed the non stock PCV (AC789C) in a 73 L-82 for a new and correct PCV (AC736C). The car was running fine with the old and runs very rough with the new 736C. What must I do to get the engine to idle smootly with the new PCV? I put the old non stock valve back on and its idles fine. I might add that the smog pump/emissions control feature is currently off the car. Would this affect PCV selection (meaning I'm better off with the 789C)?

Thanks

[Joe L.]

Re: PCV Valve switch

Charles-----

You've swapped out one incorrect valve for another incorrect valve. The valve used for your application is a CV746C, aka GM #6484525. Will this absolutely solve your problem? I don't know, but it's the correct valve for your application.

[Duke W.]

Re: PCV Valve switch

I ran across a possible similar situation. Someone had installed a "correct reproduction" valve and the car did not idle correctly. I have a suspicion that someone is selling valves that are stamped with a "correct number", but they do not have the same flow rate as original.

I have seen the same issue with vacuum advance controls. The bracket was stamped with a correct OE GM number, but the VAC performance was nowhere near original.

The PCV valve is designed with a certain flow-manifold vacuum specification, and the carburetor is calibrated to this flow rate. A significant deviation from the correct PCV flow characteristics may not be compatible with the carburetor's idle adjustment speed and mixture range, which means the engine may not idle properly.

The most important priority is to have a correctly performing valve, so beware of "reproduction parts".

My 1991 Delco catalog lists the CV746C for "1973 350 H.P.", which I assume means "high performance". My GMPD P&A catalog also shows the same AC number for '73 L-82, and the original GM long number is 6484525, I suggest you cross reference these numbers at some other brand web sites like NAPA, SMP, and BWD. Buy one, and see if it performs properly.

Note that the prefix for all AC PCV valve numbers is "CV" not "AC".

Duke

[John D.]

Re: PCV Valve switch

Duke, This is an echo answer to yours. I agree with you 100%. The restamped VA's are way out of spec. Good for fooling the judges but that's about all. Don Baker tested a pile of them on his Allen distr machine and they all flunked. Way out of specifications. Better off putting a B something vac adv from NAPA, etc. JD

[Charles P.]

Re: PCV Valve switch

Okay, I was looking in my second edition JG which lists the CV736c valve as the correct valve for L48 and L82. I assume now this is an error and don't know if this was corrected in the newer 3rd edition. I purchased 2 of them (wrong) today from chevy (PN 6423695) in the AC Delco box. Curious, I just now pulled my Chevrolet Parts and Illustration Catalog (New York phone book size) down and you are right. It lists the CV746c PN 6484525 as you say. It lists the application for the 736C as 69-72 (exc HD 427, 454, SP. H/PER.)

If the 746C does not work (poor idle again) AND it is a genuine AC DELCO part what would this be a function of (ie carb, timing, advance etc.)? Just wondering. Thanks.

[Charles P.]

Re: PCV Valve switch

As a footnote I did pick up the cv746 PCV today. I installed it and the car did run much better but did not quite as smoothly as when the 789 was in it. I wil leave that in there until I (dare) adjust the carb which I think is just asking for trouble. Ever at that crossroads? You know the one you passed when 2 days later you look back and wish you left something alone.