Setting Up and Adjusting Idle

[omaha]

Well, think of it this way. All a pcv valve consists of is a spring, a ball and a bypass passage. Oldsmobiles were noted for there smooth operation which, was attributed to their camtiming, timing, etc,etc. From the factory, they made alot of vacuum, maybe 18 to 20 inches sometimes. So they had a PCV valve that went along with this, it matched the engines vacuum. Now you have a cam that has lowered this reading so the PCV does not function correctly. (or maybe functioning intermittently) Maybe the vacuum is not enough to overcome the spring or close the passage or maybe the narrow cam is causing pulses in the intake (whatever). So basically, now you have an open large vacuum line, not a good idea if you want a good signal for a good idle quality.
    No, you do not need the PCV and you can run the setup that is sold through summit that has the oil collection resevoir (you empty it once in a while). It is like a breather that works through a hose and collexts excess oil so it does not make a mess. Alot of guys running Big block chevies with big cams cannot run a pcv either. many racers dont either.
   I will try and find the actual name of the deal that I am trying to explain.
Again, make good use of that vacuum gauge. Anything you can do to increas the vacuum is a step in the right direction with what you have. 

[defiant1]

    No, you do not need the PCV and you can run the setup that is sold through summit that has the oil collection resevoir (you empty it once in a while). It is like a breather that works through a hose and collexts excess oil so it does not make a mess. Alot of guys running Big block chevies with big cams cannot run a pcv either. many racers dont either.
   I will try and find the actual name of the deal that I am trying to explain.
Again, make good use of that vacuum gauge. Anything you can do to increas the vacuum is a step in the right direction with what you have. 

So does the canister suck the vapors out of the crankcase and still allow me to keep a high level of vacuum?  Just don't want to spend money on something and not gain anything.

[defiant1]

Cliff,

Can the gasket between the base plate and main body of the carb affect the vacuum??  Had someone tell me this based on the fact that there was an issue when the PCV was hooked up to the port on the base plate.

Are there different gaskets for this application or are they all the same??

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[Cliff Ruggles]

The carburetor throttle body is designed so that any PCV or power brake port will use vacuum from both sides of the intake manifold, no matter which gasket is used under it.

There may be some idle quality differences by using an open gasket on a fully divided intake, vs a 4 hole gasket, but the difference would be minimal.....Cliff

[jamesF]

Are there custom PCV valves?

I was aware of the PCV affect on the system by way of reading and talking to guys. So when the time came,  to put in the PCV, I asked for a PCV for a 1970 455 GTO. I figured that a PCV designed for a big cammed 455 from the factory might be appropriate in my car.

I'm driving a '75 Formula powered by my homebuilt 455. Engine doesn't suck any oil and I don't have any evidence of a "vacuum Leak" type of situation. Smooth Qjet idle at about 750 in gear when warm. 11" - 12" of Vacuum.
I am currently running a .510 lift cam with a 110 LSA.

You can read and see talk of people using different PCVs dependent on the CAM and setup. So it makes sense that PCVS are sprung differently and the flow is different depending on the car.

"The biggest difference between valves is the orifice size through the center of the valve which determines flow when the vacuum is fairly low. It has to be balanced between too big (essentially a vacuum leak) to too small (which means no flow through the crankcase).

Factory PCV valves will have varying orifice size with engine size and camshaft specs. If you have a big cam, get a PCV valve for an engine that has a fairly radical factory cam. some say vacuum is highest at idle. That is not always true. Vacuum will pick up at higher engine speeds as long as the load is low, like cruising. It's highest when you are decelerating."

Here is some info on PCV's. http://www.aa1car.com/library/pcv.htm

[defiant1]

Good info on the PCV valve above.

Update to my situation:

Well I tried just installing 2 breathers and I had a lot of blow by. So, I installed 1 breather passenger side and one breather with a tube on the drivers side. I installed the PCV valve in line between base of carb and breather w/tube.

 

Instead of focusing on initial timing I set the timing based on mechanical advance. I set it at 33 degress all in before 3100 rpm. I checked my initial timing at this point and was at 13* @ 1100 rpm. It did not want to idle well at this point. So, I increased the timing to 15* and adjusted the fast idle and idle mixtures screw accordingly. So now I am at 35* all in before 3100 rpm. The only thing I don't know is the vacuum reading at this setting (still working on installing a permanent vacuum gauge).

 

Oh, and I wound up using the stock mechanical advance springs (sort of heavy-but not as heavy as the after market ones) and the aftermarket mechanical weights.

 

It runs the best at this setting, I still have an erratic idle while in drive with brake depressed and also while going reverse at slow speed. It idles around 750 rpm while in drive or reverse.

 

So I will keep tinkering with idle mixture screws and the throttle flaps.

 

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[defiant1]

I am still trying to figure this carb out 

Pulled the plugs and they are very white, indicating a lean condition.  The plugs have over 300 miles on them.  What can I adjust to rectify this issue?  What setting needs to change to richen up the mixture?

I am still having idle issues, but no nozzle drip.  Which is good I guess. 

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