Vacuum Check Valve
Thread Starter
Burning Brakes
Joined: Sep 2006
Posts: 851
Likes: 10
From: Palm City, FL
Vacuum Check Valve
How many inches of Vaccum is needed to get a good flow of air through the Vaccum System's Check Valve????
The Replacement Valves I've been getting are very hard to get air to flow through them... almost as if you need a good amount of vacuum just to get the valve to open and allow air to flow though.
The Maximum Vacuum I was able to achieve in setting my carb, was about 13 inches. (Idle RPM is about 850-1000 and I have a lumpy cam)
Is 13 inches enough to get the valve to open enough for good flow?
The result of the new check valve is that the system seems to lack the Vacuum it needs to get the lights and wiper door to open and close at a normal rate.
With the old Check valve (which is broken and allows air to flow in both directions)... the flow through the valve is very easy, and there is enough vacuum in the system for everything to work correctly.
The problem I have with keeping the old broken valve is that when I shut off the car, the vacuum is lost back our though the intake, and nothing works unless the engine is running... no vacuum is able to stay in the reserve tank.
Does anyone have any info on these check valves and how much vacuum is needed in order for them to work right?
The Replacement Valves I've been getting are very hard to get air to flow through them... almost as if you need a good amount of vacuum just to get the valve to open and allow air to flow though.
The Maximum Vacuum I was able to achieve in setting my carb, was about 13 inches. (Idle RPM is about 850-1000 and I have a lumpy cam)
Is 13 inches enough to get the valve to open enough for good flow?
The result of the new check valve is that the system seems to lack the Vacuum it needs to get the lights and wiper door to open and close at a normal rate.
With the old Check valve (which is broken and allows air to flow in both directions)... the flow through the valve is very easy, and there is enough vacuum in the system for everything to work correctly.
The problem I have with keeping the old broken valve is that when I shut off the car, the vacuum is lost back our though the intake, and nothing works unless the engine is running... no vacuum is able to stay in the reserve tank.
Does anyone have any info on these check valves and how much vacuum is needed in order for them to work right?
Last edited by genuine1980; May 29, 2013 at 12:25 PM. Reason: e
Le Mans Master
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Thread Starter
Burning Brakes
Joined: Sep 2006
Posts: 851
Likes: 10
From: Palm City, FL
Drifting
Joined: Jul 2006
Posts: 1,259
Likes: 154
From: Deplorableville Nebraska
You have more than enough vacuum. Do you have a NAPA store near you? If so, buy part #CRB 2971 vacuum check valve, and install it between the filter and your OLD check valve. You will now have the split vacuum source to each of your circuits, and the added benefit of a check valve that works.
Think outside the box.
Think outside the box.
Thread Starter
Burning Brakes
Joined: Sep 2006
Posts: 851
Likes: 10
From: Palm City, FL
You have more than enough vacuum. Do you have a NAPA store near you? If so, buy part #CRB 2971 vacuum check valve, and install it between the filter and your OLD check valve. You will now have the split vacuum source to each of your circuits, and the added benefit of a check valve that works.
Think outside the box.
Think outside the box.
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Thread Starter
Burning Brakes
Joined: Sep 2006
Posts: 851
Likes: 10
From: Palm City, FL
I checked for leaks.... and I tested the leak theory for the carb and idle problem by plugging the vacuum system at the intake manifold to eliminate that possibility.... still had the same idle.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,116
From: Crossville TN
There should be no problem blowing though the check valve in one direction and sucking to get airflow from the other direction. But, reverse it and it's works backwards!!! Amazing, huh??? 
Melting Slicks
Joined: Oct 2002
Posts: 2,317
Likes: 111
From: St Louis MO
Your engine should have plenty of vacuum to pull through the check valve. If not at idle, raise the rpm to a fast idle and you'll probably see over 17" Hg.
If your headlights are sluggish, your vacuum servo valve is probably on its last legs.
If your headlights are sluggish, your vacuum servo valve is probably on its last legs.
Race Director
Joined: Jul 2001
Posts: 15,892
Likes: 42
13" is plenty. The headlight system will work down to about 7ish and the check valve itself should open with about 1" or so. Try to find out what's wrong with yours.
Thread Starter
Burning Brakes
Joined: Sep 2006
Posts: 851
Likes: 10
From: Palm City, FL
I agree... the check valve 'should' have a good flow with 1" or so. I am finding that it does not have a good flow... it is very difficult to suck air through it... I tried 2 replacements and both the same. Very difficult to suck air through. As if it is taking a lot more than just 1" to open it.
Thread Starter
Burning Brakes
Joined: Sep 2006
Posts: 851
Likes: 10
From: Palm City, FL
The two OEM replacements I got both allowed no flow in one direction as expected obviously... but in the direction in which air should flow freely, it is very difficult, and little air flows through. It is then when I have the new replacement OEM check valve on that the vacuum system accessories are very sluggish.