Carb question again

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statsystems
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Carb question again

Post by statsystems » Mon Oct 30, 2017 10:12 pm

I'm fighting my tune up. I want to run a PCV valve. And I think it's causing issues.

I just pulled the PCV valve from the valve cover and there is no flow through the valve. So I pulled it off the hose. There is virtually nothing moving through the PCV valve port. It idles at 900-950 and has ~ 10 inches of vacuum at that idle speed.

I'm stumped. It would seem that even at that low vacuum I should still have some air movement through the hose. And I think I should see the idle speed slow down when I plug the PCV valve hose and speed up when I unplug it.

Cam is 281/281 advertised 255/255@.050 and gross lift of .620 on a 105 at 105 on 340 CID. So I get why the idle vacuum is on the low side. I still think there should be some air movement through the PCV valve at idle.

Am I nuts or is something wrong.

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Re: Carb question again

Post by tresi » Mon Oct 30, 2017 10:42 pm

Get you some adaptor fitting so you can hook your PVC vacuum line to your gauge. It should read full manifold vacuum. If not the port has become plugged. I've seen them plug if they ever been run capped off.

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Re: Carb question again

Post by statsystems » Mon Oct 30, 2017 10:56 pm

tresi wrote:
Mon Oct 30, 2017 10:42 pm
Get you some adaptor fitting so you can hook your PVC vacuum line to your gauge. It should read full manifold vacuum. If not the port has become plugged. I've seen them plug if they ever been run capped off.
Brand new carb and I made sure it was open. I'm checking vacuum off the front of the carb. It's a ProForm carb so it has two small ports up front. I looked real close and one looks like it may be timed, and the other one is for sure full vacuum.

I have an adapter to make it read vacuum off the port in the rear. Do you think it's worth it?

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Re: Carb question again

Post by DaveMcLain » Tue Oct 31, 2017 9:41 am

When you pull the PCV hose off of an engine it should flow enough that the engine will not stay running. If not it shouldn't be hard to find the problem. Many times people will use fuel hose to go to the PCV and this never work well. It is much better to use a stiffer hose like some that's made for AC or air hose so that it can't collapse.

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Re: Carb question again

Post by statsystems » Tue Oct 31, 2017 10:45 am

DaveMcLain wrote:
Tue Oct 31, 2017 9:41 am
When you pull the PCV hose off of an engine it should flow enough that the engine will not stay running. If not it shouldn't be hard to find the problem. Many times people will use fuel hose to go to the PCV and this never work well. It is much better to use a stiffer hose like some that's made for AC or air hose so that it can't collapse.

Exactly. I pull it off and zip. NADA. Nothing. I know the hose and PCV valve aren't plugged because I can blow through them (and yes, both tasted bad) and the carb is a BFN ProForm and I made myself look to see that the PCV port was drilled all the way through.

I'm just stumped. Even with 8-10 inches of vacuum at idle I should still have air flow through that port right?

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Re: Carb question again

Post by F-BIRD'88 » Tue Oct 31, 2017 11:03 am

Good call on the PCV valve vacuum hose quality. Yup heat can make a crappy hose collapse.

With that big cam you may need to fool around with various PCV valves to get the PCV valve internal pindle valve flow action correct at idle. The idle vacuum has to act on the internal spring correctly to switch it from low to high flow correctly for idle ing and for part throttle driving too. 2 different modes.

If you are handy you can take a common OEM PCV valve apart and "hack" it modify the spring and the bypass flow rate orifice to get the flow rate and action you want.
Some people just go to a simpler single action set orifice type valve ( GM LS truck motor replacement) and fool with the internal orifice size to get the flow rate they want. it is a simpler design PCV valve but still has the anti back fire check valve action inside.

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Re: Carb question again

Post by statsystems » Tue Oct 31, 2017 11:21 am

F-BIRD'88 wrote:
Tue Oct 31, 2017 11:03 am
Good call on the PCV valve vacuum hose quality. Yup heat can make a crappy hose collapse.

With that big cam you may need to fool around with various PCV valves to get the PCV valve internal pindle valve flow action correct at idle. The idle vacuum has to act on the internal spring correctly to switch it from low to high flow correctly for idle ing and for part throttle driving too. 2 different modes.

If you are handy you can take a common OEM PCV valve apart and "hack" it modify the spring and the bypass flow rate orifice to get the flow rate and action you want.
Some people just go to a simpler single action set orifice type valve ( GM LS truck motor replacement) and fool with the internal orifice size to get the flow rate they want. it is a simpler design PCV valve but still has the anti back fire check valve action inside.
I'd was going to order the M.E. Wagner dual flow adjustable PCV valve but if I have no flow at all how would an adjustable valve, or even changing the flow rate on an OE type valve?

That's what's driving me berzerk. This doesn't make sense. Or it isn't making sense to me anyway.

Should you not be able to pull the hose off the back of the carb and have it act like a giant vacuum leak? Or just pull the PCV valve from the cover and hold your thumb over it and make the engine speed slow down because you just stopped an air leak?

I think I've lost my marbles.

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Re: Carb question again

Post by F-BIRD'88 » Tue Oct 31, 2017 11:29 am

Yes, if you pull the 3/8" PCV valve vacuum hose off the base of the carb while its running there should be a big vacuum leak. If there is nothing the passage or the fitting is blocked.... Could be a blocked hose too.
Something inside it or it is kinking when hot.

Those carbs may/Or may NOT also have a auxiliary idle air bypass for extra air at idle that is right under the air cleaner stud hole. you'd have to look.
Maybe a big fat cricket or a lil mouse moved into the PCV vacuum hose.

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Re: Carb question again

Post by DaveMcLain » Tue Oct 31, 2017 11:33 am

statsystems wrote:
Tue Oct 31, 2017 11:21 am
F-BIRD'88 wrote:
Tue Oct 31, 2017 11:03 am
Good call on the PCV valve vacuum hose quality. Yup heat can make a crappy hose collapse.

With that big cam you may need to fool around with various PCV valves to get the PCV valve internal pindle valve flow action correct at idle. The idle vacuum has to act on the internal spring correctly to switch it from low to high flow correctly for idle ing and for part throttle driving too. 2 different modes.

If you are handy you can take a common OEM PCV valve apart and "hack" it modify the spring and the bypass flow rate orifice to get the flow rate and action you want.
Some people just go to a simpler single action set orifice type valve ( GM LS truck motor replacement) and fool with the internal orifice size to get the flow rate they want. it is a simpler design PCV valve but still has the anti back fire check valve action inside.
I'd was going to order the M.E. Wagner dual flow adjustable PCV valve but if I have no flow at all how would an adjustable valve, or even changing the flow rate on an OE type valve?

That's what's driving me berzerk. This doesn't make sense. Or it isn't making sense to me anyway.

Should you not be able to pull the hose off the back of the carb and have it act like a giant vacuum leak? Or just pull the PCV valve from the cover and hold your thumb over it and make the engine speed slow down because you just stopped an air leak?

I think I've lost my marbles.
if you pull off the hose it should be a vacuum leak big enough to cause the engine to die. If not then you're on to where the restriction in the system is located.

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Re: Carb question again

Post by stealth » Tue Oct 31, 2017 11:39 am

I can only urge you to reconsider the use of PCV ...

I have seen nothing but bad things..

Oil soaked crusty intake valves
oil soaked plugs (especially a port that gets most of the air)
ONE or two ports tends to get most of the oil/air mixture
Just plane trouble... (for lack of other term)

what is it you like about the PCV that makes you want to run one?
Last edited by stealth on Tue Oct 31, 2017 11:39 am, edited 1 time in total.
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Re: Carb question again

Post by F-BIRD'88 » Tue Oct 31, 2017 11:39 am

Probably was a piece of junk in the vacuum hose and it is now stuck in the 3/8" hose fitting tube on the base of the carb and blocking vacuum flow there.

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Re: Carb question again

Post by F-BIRD'88 » Tue Oct 31, 2017 11:43 am

stealth wrote:
Tue Oct 31, 2017 11:39 am
I can only urge you to reconsider the use of PCV ...

I have seen nothing but bad things..

Oil soaked crusty intake valves
oil soaked plugs (especially a port that gets most of the air)
ONE or two ports tends to get most of the oil/air mixture
Just plane trouble... (for lack of other term)

what is it you like about the PCV that makes you want to run one?
Get some valve cover that have PCV breather oil baffles THAT WORK and the breather holes located in the right spots on the top of the valve covers and you won't see this.

Most of the aftermarket valve covers are crap. No baffles and or holes in the WRONG spot.

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Re: Carb question again

Post by statsystems » Tue Oct 31, 2017 11:46 am

I'm going out in a bit and look and see if something is in the hose. But I can take the hose off completely and it changes nothing.
And I know the carb isn't blocked back there. I looked at it physically when I bolted it on yesterday.

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Re: Carb question again

Post by stealth » Tue Oct 31, 2017 11:48 am

Well that may be... but I've had trouble with quite a few

Then I considered the question of what exactly was I trying to achieve by running a PVC... why risk having these issues?

And the really cool looking covers don't have baffles.......... :D
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Re: Carb question again

Post by Tuner » Tue Oct 31, 2017 12:01 pm

PVC is Poly Vinyl Chloride, a form of plastic.

PCV is Positive Crankcase Ventilation. PCV is a good thing because it reduces the amount of water and fuel that otherwise accumulates in the oil and forms acids and other nasty things.

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