I am aware that manifold vacuum is responsible for crankcase ventilation from valve cover to manifold. I am trying to understand it better for tuning. I am currently running an AC PN CV789c PCV valve. The correct valve for my 73 l82 is a 746c. The correct valve (746) causes a rough idle when I put it on. Is it too much vacuum or too little that causes rough idle and is there a simple carb adjustment I can make to even out the roughness with the 746 PCV on the car? Also does anyone know if the numbers reflect flow (ie a higher number equalling less or more flow)?

 

OK since no one has offered an answer yet, I'll take a swag. It sounds to me like the PCV valve is not opening given the amount of vacumm your engine is creating. How much vac do you have at idle? So, you either need a PCV valve that opens with less vacumm or you need to tune your carb or adjust timing to raise vac up to a respectable level. Should be 10-14 inches of vacuum? Again, this a simple man's guess.

 

Good idea. Anyone know where can I get a cheap vacuum gauge? Is it a simple adjustmet to increase the amount of vacuum? The timing was just checked and is at spec 8 degrees.

 

Sears
Harbor Freight
Most autoparts stores

All about the same price.

 

the 2 pcv valves we use for performance engines are AC/DELCO #s CV769c and CV774c The CV774c has a 90 degree plastic elbow on it while the CV769c does not.
Henry @ olescarb

 

I use the same PCVs as Henry described above - I find the 90-degree unit to be most versatile for a clean hose routing.

Although there may be slight flow differences between PCVs, any PCV will allow flow even with minimal engine vacuum. You don't have a vacuum issue, and a vacuum check will not give you any usable data for PCV selection. No, you cannot simply change your engine vacuum - vacuum is a function of engine efficiency based on your cam profile and other fixed engine parameters.

Your timing is way off. Fix your timing. You need to be at 36 total, which will likely put your initial up into the low-to-mid teens. Run your vacuum advance to a manifold vacuum source once you have fixed the base timing. You don't want 8 degrees initial timing unless you're trying to pass an emissions test. Your idle quality will likely be fixed once you fix the timing.

 

the CV789c is for a 2.8 v-6 chevy, it is a VERY low flow pcv valve. the letter on the bottom of the pintle valve inside the pcv valve indicates the size of the pintle, but i know of know why to rate find the flow rate other than open each box and look at the letter on the pintle. I think this letter rating only applies to AC/DELCO brand pcv valves but i could be wrong.

Henry @ oles

 

On this topic, here's a neat little tuning thing you can do:

The PCV that Henry and I use (referenced above) is a modest-flow PCV. This is good to use on a performance engine, since the last thing you want is a lot of uncontrolled air dilution with the resultant lean-out condition (a PCV is, after all, just a controlled vacuum leak). But most carbs can easily be set up and tuned for the PCV in use.

But if you have any questions about the PCV "leak" sizing on your particular engine, you can simply start drilling a few 1/16" diameter holes right into your PCV vacuum hose going to the carb. The rubber hose is, obviously, cheap and easy to replace. So drill a few holes in the hose and see how the engine reacts in terms of idle quality. If it idles better with a few holes, you might want to use a PCV with a higher flow rate. Or... you may want to lean out your idle mixture a tad.

The other thing you can do with this "tuning method" is to see if you really need to drill your carb throttle plates: You have heard of the need to drill the carb throttle plates on big-cammed engines in order to get enough airflow with the throttle plates closed to obtain not more than .020" transition slot exposure. Before you trash a set of throttle blades by drilling them for more "air bypass," simply drill a few holes in the PCV hose to see if it produces the desired results. If it doesn't, there is no need to drill your plates and ruin a perfectly good carb...

 

The car came from New York State originally. Maybe they were trying to pass emissions there. I can't imagine why they selected that PCV because the engine was totally rebuilt and it seems like a funny error to make if it was an error. Maybe their thinking is the lower the better for performance?. At any rate it runs super with the 789C but I am a bug and did not like the 90 degree angle PCV since it does not look like my other cars setup which is all original and has no bend in its route to the carb. So if I understand Lars correctly I need to increase my initial timing to the low teens from 8 degrees if I want to try to use the 746 PCV. Otherwise I may take your advice and go and get the CV769 which you say has no bend and is a LOW flow good for a performance engine but not as low as the VERY low 789.

So i will set initial timing first to low teens then rehook the vaccum advance. I don't understand what you mean by "to a manifold source". Do you mean to a place other than it is now? I guess what you mean is I am looking for a way to get 36 degrees total?

 

LARS,
On this same subject, from the charcoal cannister comes two lines, one goes to the pcv valve and then continues to the base of the carb, this is the large hose, the second line goes from the canister to directly to the carb and is the small hose. My question is should the small hose go to a ported vacuum sorce or a full time vacuum source? I am using a holley carb on the 71 LS6?

 

ttt

 

Lars Ttt

 

ttt lars!!!!!!!!

 

Try a PM or if you need it I have an email

 

What they're trying to say is that you need to set your timing to 36* with the engine running at 2,800-3,000 rpm with the vacuum advance hose disconnected and plugged. Let the idle timing fall where it falls. It may be 12*, it may be 16*, it doesn't matter as long as your total timing is 36*. Then connect the vacuum advance hose to a port on the carburetor that has manifold vacuum at idle.

 

For future reference - Ported

 

Read Barryk's timing post in the sticky at the top of the Tech page. Everythink you need to know about timing.

 

The manifold source means to a fitting that is at manifold pressure, which is *below* the butterfly valves in the carb, or anywhere in the manifold. Using a fitting in the carb throat will disperse the oil vapor among the pistons. The ported fittings are located somewhere above the butterflies and have less and differing vacuum. Thinpockets picture shows this pretty well.

 

Sorry I didn't see your post until now. If you need me on specifics, drop me an e-mail or a PM...

Yes, the charcoal canister vent line goes to ported vacuum. By doing this, you avoid what effectively becomes a massive vacuum leak at idle: The ported source does not pull any (or much) vacuum at idle, so you do not get any adverse intake charge dilution at low engine speed. Once the engine is off idle and operating at higher rpm, the ported source will allow venting of the canister with a slight dilution of the intake charge. This dilution becomes irrelevant at the higher rpm. Under power, since there is no manifold vacuum at WOT, the system does not function, so there is no WOT power penalty. So overall, it's really not a bad system, and it does not rob any power or drivability from the engine.

 

What is purpose of the charcol cannister? Mine isn't connected. Should I reconnect it? I am running an Edelbrock Performer manifold and 1406 carb?