Holley 390CFM tuning 258

The CCV system is interesting. No PCV valve, just elbows on the valve cover. The back elbow sucks out valve cover gasses with manifold vacuum. The front elbow replaces that with fresh air from the intake.



The front and back elbows are different. The back one is "metered" meaning it has a small hole in it to minimize the amount of vacuum taken from the intake manifold.  The front elbow is full size.



I have seen some people use breathers instead of plumbing back to the intake.
I think the breather can work if it's providing enough restriction. The chrome breather I had in there was not super tight in the gromet and the filter had almost no restriction. When I sprayed it with a bunch of carb cleaner, it provided more restriction and the vacuum leak was minimized and the idle seemed to decrease because there was less air getting into the engine.

Do any of you have an idea of which would be better for tuning with the carb?
The breather would be "leaking" in air according to the filter resistance and manifold vacuum.
The CCV system plumbed back to the air intake would have a more complex relationship as it would relate to air intake suction AND manifold vacuum which respond differently over the RPM curve.
So the breather might be easier to account for in tuning the carb but it's also different than stock which is a proven system (but it probably goes back to the intake simply for smog reasons.)

I'll try the breather first since it's easier to setup.

The grommet is your problem, but you'll need the correct valve to get the orifice to reduce the vacuum draw. Replace both and it should solve that issue. You will never get the carb right if you can't control the vacuum leakage.

PCV is by design a controlled vacuum leak, just small enough to do the job. A breather is exactly the same thing as connecting to intake, just a source of filtered air. You can do it either way same result.

Hesco sells that grommet and valve for the cover in the picture, pretty cheap. It's a standard part anywhere will sell it I think.

Pdok wrote: The grommet is your problem, but you'll need the correct valve to get the orifice to reduce the vacuum draw. Replace both and it should solve that issue. You will never get the carb right if you can't control the vacuum leakage. PCV is by design a controlled vacuum leak, just small enough to do the job. A breather is exactly the same thing as connecting to intake, just a source of filtered air. You can do it either way same result. Hesco sells that grommet and valve for the cover in the picture, pretty cheap. It's a standard part anywhere will sell it I think.

I put in a better breather and that increased vacuum. I will try putting a needle valve inline to the intake vacuum so I can turn that off or adjust it. If I turn off vacuum to the valve cover ventilation, it will still exhaust passively.
I'll try this tomorrow.

On my 4.0 valve cover, I replaced the the metered fitting at the back of the valve cover with a push in style pcv valve. I've had no issues, I can dig up part numbers if it helps you. I think it was off a chevy 250? Don't give up, it will be worth the effort in the end! As Farna say's, take it one problem at a time.

Replace the ball valve with an in-line PCV valve. That will probably solve the problem.