Technical Vacuum Nightmare

Hello all, long time reader first time caller. (I hope I posted this correctly)

Earlier this year I bought a known running 396 ,and I have been driving myself insane trying to solve why this thing has hideously low manifold vacuum pressure.
When I purchased the engine the previous owner had efi on it, and he was looking to put it on another engine. So he removed the throttle body and intake. I wanted to go carbureted anyway so this wasn't a big deal. I swapped the intake and threw thew on a Holley carb, and I couldn't get it to idle below 1500 rpm at about 13lbs of manifold pressure. So I replaced the gaskets and tried again. Same results... So long story short. I have tried multiple intakes about 10 different gaskets. Rebuilt the carb. And even had the heads gone through. After all that at it's current state it will idle around 750 ish rpm but the manifold vacuum is still super low at around 6lbs. I checked for compression, the valve timing, and the valve lash. I have sprayed carb cleaner around the intake and carb to see if its leaking and nothing. Everything seems to be correct. My only guess is something weird is going on with the carb but I am at my wits end. So if anyone has any advice or tips. That would be much appreciated. Thank you in advance.

As for specs
Holley xp carb, air gap intake, stock heads, cam unknown. Previous owner said it was mild. Aftermarket roller rockers, msd ignition box, And it's been bored to a 402. Thanks again

 

The cam might be bigger than you think. That would do it.
Check to see what power valve is in the Holley. The power valve opens at low vacuum levels to dump in more fuel. If the vacuum at idle is 6 and the power valve opens at 10, it will idle pig rich. You need a power valve with a setting that is lower than the vacuum at idle.

Also with a big cam, you need at least 12 degrees of initial timing, checked with the vacuum advance disconnected. 16 would be better. That will help the idle. But the distributor may need to be recurved to keep the total advance where it needs to be.
Once you have the initial timing set, hook up the vacuum advance to manifold vacuum. That will also help the idle.

 

It would be very helpful to know the cam specs. It might be as smooth as it gets. Efi will tolerate more cam overlap than a carb. Because vacuum signal is just about irrelevant with efi, but it is essential for a carb to function.

If it doesn't have a degreed damper, you can get a tape to put on it that is marked in crankshaft degrees. Then use a piston stop to make sure that tdc is properly indicated on the damper.

Then you can take off the left valve cover and set up a dial indicator on the valve retainer of the #1 intake valve. Then you can measure valve lift, and plot out duration. (Repeat on the exhaust if you're curious.)
Some cams just won't idle smooth.

 

Which stock heads, and does the manifold properly match the ports?

 

Stop throwing things.

 

Yeah. Don't bring violence here.

 

Unlikely I know with what you have done. There isn't a large vacuum port open at the base of the carb at the rear?

 

Secondaries maybe not closed?

 

One thing, DO NOT ***UME, you have to positively know that anything you use including the carb is in proper working order.
Is the carb a known carb that has been run with success on something else recently?
Have the heads been surfaced possibly to the point where the intake (which ever one) no longer will seal correctly? That doesn't happen often but if someone went wild surfacing the heads in the past it could throw things off. It works if the intake is milled to match but a stock spec intake might not sit down on the gaskets square. If you have one of the sets of gaskets you had on and off there look them over to make sure that they were sealing all the way around the ports, make sure that the imprint from the intake is even. If it isn't sealing right the gasket will be compressed a lot more at the top than at the bottoom.

 

Also, a few things are off here.

You aren't measuring manifold pressure.

You are measuring manifold vacuum.

That is not typically measured in pounds-per-square-inch, but in inches-of-Mercury.

Is your gauge in PSI?

One PSI is 2.03602-inches-of-Mercury.

 

Also, not all modern carburetor cleaner is flammable.

Check your can.

If it says non-flammable, it's useless.

 

I wasted 5/6 bucks on a can year or so ago, couldn’t find a vacuum leak. Took the truck in for a smoke test, guy say my intake gasket were munched.
When I got home, looked at the cam, non-flammable. 5/6 dollar can cost me a 70/80 $ smoke test.

 

Just use an unlit propane torch for hunting leaks - much easier to pinpoint with the tip.

Frank

 

I like this test. No air cleaner, vacuum hoses plugged as applicable, let it idle. Put your hand over the carb air horn and see what it does. If the idle picks up and it smooths out, you have a vacuum leak somewhere.

 

Thank you, that was bothering me too. Although I used to work with a guy that was a certified airfram mechanic, and he always laughed when somebody talked about vacuum, "it's not a vacuum, it's a pressure differential". Technically there is still pressure in the intake manifold, though it may be below atmospheric pressure, it is still a positive pressure vs a true vacuum. What we measure is the difference between the pressure in the manifold and atmospheric pressure, and we call it vacuum. Airplane guys call it manifold pressure.

 

For starters, unplug every vacuum line one by one, and close the inlet with your finger, just to make sure you don't have a vacuum leak at the brake booster, a stuck PCV valve or anything.

Frank

 

I don't believe Holley XP carbs come with any vacuum ports, more of a race style . Wouldn't have been my first choice for a street car.

 

Vacuum = map= manifold absolute pressure.

Ben

 

Very true, but either way, using the wrong scale to measure whatever we want to call it, vacuum, pressure differential, depression, or whatever, the number is not going to reference the commonly accepted scale.

All of my equipment, digital, or ****og, hot rod, or German sports car, when in US mode, says in/HG.

 

He said psi pressure, but I knew what he meant. I like bobss396's idea of the hand over the carb. Quick, easy, free.

 

If there is no pressure in the intake it has to be vacuum below 0 psi

 

The point is that there IS pressure in the intake, it is below atmospheric pressure, but it is still a positive pressure above o. It would have to be completely sealed off in order, not open to atmosphere at all, to reach a state of true vacuum; and of course the engine wouldn't run like that. The bottom line is, it's not a true vacuum.

 

I've used some gauges and worked on some equipment where the measurement is in/H2O (the gauge has actual water in a tube next to a ruler); And you can convert that to in/HG if needed.

 

Yes, Ford's famous Variable Venturi Carb had adjustments made with a vacuum gauge measured in inches of water. I have a very infrequently used one in my tool box.

 

heh...I just checked, my 396 is making 6 inches of vacuum idling close to 1000 rpm.

what are you *****ing about?

If you want an engine with good ****, get an old Rambler or something.

 

Lol I have to throw in my two cents. There is vacuum in the intake on gasoline cars because they run what???? ( ****erfly’s on the carb) this creates the vacuum (inches of Mercury) to pull the atmospheric gas from the bowls to the Venturi !!!!! ( from the bowls that are at atmospheric pressure all the time) this is why carbs are vented in the bowl. Only diesels run no throttle plate. Reason they don’t create vacuum. Psi and inches of Mercury are two totally separate measurements.

I know it’s hard to understand over text. If you have time look up the theoretical draft of water.

No chance of psi developing in an intake and run with a carb ( with no boost) turbos and superchargers change everything.

some one please correct me if I’m wrong.

 

Must really ****...

 

Yeah, you're wrong. The ****erflies are never closed all the way, air needs to move through the carburetor or the engine will not run. As I've posted several times already, and starting to sound like a broken record, the pressure is less than atmospheric, but above a true vacuum. Gas is pulled into the venturi due to the venturi effect, not because there is a vacuum in the intake manifold.

 

You are referring to MAP, is your gauge have PSI on face? If a correct vacuum gauge, should be (in. Hg. ) Some MAP gauges ( with psi on face) calculate reading with atmospheric psi ,14.7, then add your engine vacuum of , 19hg = 4.3 psi. Map sensor on new car works same way.

 

Our German word for it is "Unterdruck" (under pressure) - I think it's a good compromise