PCV just doesn’t seem right

[DC_Gearhead]

I haven’t been on for a while but I’m back now and I’m struggling with something that doesn’t make logical sense to me. PCV valve.

From what I read on here and other forums, I’ve plumbed it right. New PCV valve in my passenger side valve cover with new tight grommet. Yes I shook the PCV valve to see if it rattles. I ran new 3/8 hose from the valve to the full vacuum at the carb. Carb is a Demon Street and I verified the full vacuum. I’ve tuned the carb and timing to run great, but when I hooked up the PCV the idle went up…meaning vacuum leak. I pulled the PCV valve out of the grommet, but still on the hose and there is a lot of vacuum going through it at idle.

Take a look at my pics. It doesn’t seem right to lose that much vacuum. I had assumed since vacuum is higher at idle, the little weight would pull to the top and stop air until you get moving and the vacuum decreases then it lets the fumes in to burn up. At least that is what made logical sense to me.

Strange thing though. Even with the PCV valve connected and sucking air, my vacuum readings off my other port only dropped by 1-1.25 in of vacuum. I’m running at 18 with PCV blocked and right around 17 if hooked up.

Do I have the PCV valve plumbed correctly and does the above seem right?

Sent from my iPhone using Tapatalk

 

[73azbronco]

at WOT, how much vacuum do you have, very little, everything is flowing straight through engine, nothing is causing vacuum, or very little vacuum. You do have lots of velocity WOT, that moves the fuel out of the carb.. Very little is pulled through PCV at WOT, so at idle vacuum, thats when the pcv system works. There is a special carb port to pvc, not straight to intake, but metered, or restricted, sounds like its working, but maybe wrong port under carb?.

 

[DirtDonk]

You never tune up a carb/engine without the PCV connected. It's an integral part of the running of the whole system, and therefore must be fully functioning before you can tune it.
So, lucky you, you get to re-tune it.
It could be described somewhat, as a "controlled vacuum leak" if you choose to look at it that way. But anything that is going to run with the engine (vacuum advance, PCV, EGR, etc.) has to be connected during the tuning process to get things running the way you want them to run under normal conditions.

Some ports might be metered, but in many cases (and likely yours?) it's just a vacuum port. It's to a common plenum area, and not just one single intake runner like the brake booster can connect to, but in most cases, it's just a regular full vacuum port.
Is it the correct PCV valve listed for your particular engine? Made very many modifications to said engine?

Paul

 

[DirtDonk]

Oh, and the rattle test is only so accurate, and gets you only so far.
If it's a very old PCV valve, rattle or not, I'd replace it. It's inexpensive and might give you some improvement.
The one you have does not look like an original, but it might still be correct.
At the very least, you can clean them up good with some carb cleaner.

Paul

 

[Timmy390]

When the pcv system is working properly it is a vacuum leak be it a controlled one.

Make sure you hook up to the port on the carb that specific to pcv and then with pcv hooked up.

Tim

 

[Broncobowsher]

As stated, need to tune with it hooked up. That will fix everything. Some of the idle airflow is from the PCV. You tuned without that airflow and now you have it. Vacuum is down because it isn't tuned for idle anymore.

The only vacuum line you remove for a tune up is the vacuum advance. This is to get a good initial timing setting without any chance of a vacuum induced error.

 

[B RON CO]

Hi, as mentioned, the PVC valve is drawing air through the engine during high vacuum situations. Just put your thumb on the bottom of it when the engine is idling and you will see. I believe that the PCV system is is operating properly, and that you need to adjust everything, but like Paul said, make sure you have the correct PCV valve. Good luck