Manifold Vacuum

Disconnect the power brake hose and read the intake vacuum again.
It should be 15-17in-hg.
If it is with the brakes blocked, you have a leak in the break system.
Second setep reconnect the booster at the engine. Disconnect at the booster itself and read it.
If it is normal, then there is a leak in the booster itself.
You will probably need to replace it if there is a leak in it of have it rebuilt.

Doug - Thanks for your comments. I don't think I have a vacuum leak in the brake system though. I am running a cam with a lot of overlap. The brakes work fine and the engine idle speed does not change when I apply the brakes. From my experience I would expect a change in engine idle speed if there were a vacuum leak in the brake booster system. I was just wondering why there is a difference between the two measurement locations and which power valve I should use. I think I have a 6.5" valve in the carb now.

BD,

There is a difference between ported & manifold vacuum, they are never 100% equal, although they are very close at wide open throttle.

"Ported" vacuum is tapped into the carburetor above the throttle plates, at the discharge of the venturi. "Manifold" vacuum is tapped into the carb below the throttle plates, or into the manifold itself. In either case, it is the vacuum of the intake plenum.

Below is the relationship of the strength of the vacuum signals at various throttle settings:

............................PORTED....... MANIFOLD

Idle.........................none.............high

Part Throttle ...........low/mid..........high/mid

Wide Open Throttle...low...............lower

The power valve should be selected to only open at wide open throttle. The power valve is controlled by manifold vacuum. So to determine what power valve you need, you should monitor intake manifold vacuum, with the car under load (i.e. being driven on the road), and the accelerator pedal mashed to the floor.

your friend on the DTBB

quote:

Originally posted by george pence:

The power valve should be selected to only open at wide open throttle. The power valve is controlled by manifold vacuum. So to determine what power valve you need, you should monitor intake manifold vacuum, with the car under load (i.e. being driven on the road), and the accelerator pedal mashed to the floor.

Did you overstate? That doesn't sound accurate, George.

My understanding is the PV should be set 1" lower than your vacuum at cruise. This way the PV opens to richen under higher load via the PVCO's delivering roughly 25% more fuel, but it is closed while cruising for a lean, milage improving burn.

Vacuum is essentially nothing at WOT, so they all open at WOT regardless of number stamped..setting it to open at WOT is, too late.

If the power valve opens too soon, and the mid-range (part throttle) becomes too rich, the engine will blubber and/or refuse to rev.

You can't use a hard number like 1" of Hg below vacuum at cruise, because the tuning for each motor can be very different, and the difference between the vacuum at cruise and at WOT might drop several inches of mercury, or only a small amount. It depends upon how much vacuum you have to begin with.

To clarify my earlier statement, the power valve must open when you floor the acelerator pedal (WOT), but it should not open at any carburetor butterfly position less than WOT. As an example, if the intake manifold vacuum drops to 1.5" Hg at WOT, then the power valve should open at 1.8" Hg, or 2.0" Hg. If you installed a power valve set at 1.5" Hg, it would never open fully, as the vacuum never drops below the power valve's setting.

your friend on the DTBB

George I'll have to respectfully disagree

If the mains are so fat that it performs well at 3/4 throttle with the PV closed, then the mains would be too large for efficient cruise.
If it blubbers when the PV opens at anything but WOT, the PVCOs are too big, and/or the mains are set too big to cover the late opening PV and their combined fuel is too rich.

Right out of my HOLLEY BOOK ..

The solution is to choose the correct power valve and to do that you need a vacuum gauge. On a manual transmission vehicle, hook up the vacuum gauge and take the reading with the engine at idle. Then use a power valve that is rated 1-2 inches below that amount.

I suggest hook it to CARB Vacuum because thats the vacuum that will effect it.

Remember backfiring and belching blows out a power valve. Of coarse make sure that there are no vacuum leaks .. starter fluid will tell the tale.

Ron

Oh and make sure the accelerator pump levers are loose with .015 clearance, or make sure the jiggle a bit, the idle screws out about 1-2 turns and floats are set just to dribble out when you push the car with your thighs. And jetting well I usually start with stock and play with them. My advise on a stick car vacumn secondaries and change the spring as necessary after everything else is running fine. Since the operate on vacumn the rest has to be operating correctly. This info is not made up .. its practice and most of it right out of the Holley book. Dont drill nothing .. all those tricks dont work. Automatic car you could go to 50cc accelerator pumps or a real HP 351C stick car.

I plan on running the car on a chassis dyno in a couple of weeks. I will hook up a vacuum gauge when I do that and watch what the vacuum does. The chassis dyno I use measures the fuel air ratio also. Thanks for all the advice. I will keep you posted.

Great stuff BD, I love the chassis dyno. Calibrating a motor is the second best use of a chassis dyno. It was my normal practice to allow my dyno guy to tune the carb. I found this much easier than doing it by the seat of the pants myself, the extra expense was well worth the headaches it avoided & time it saved. Good luck with it my friend.

Ron & PTA, its a hallmark of this forum that members can have differences of opinion without bruising egos, being disrespectful, etc. I think that is bitchen! Thanks for that.

And to make sure I don't step on any toes, allow me to say that I understand some people enjoy tuning their carbs themselves. In saying I prefer allowing a dyno operator to do so I am not saying there is anything wrong with tuning your own carb. Not at all. For my circumstances when I worked on other peoples cars, it was too time consuming. I was a young man raising a family, time away from my children needed to be minimized. Plus the volume of cars involved meant I needed to get the cars tuned quickly, I didn't have the luxury of being able to take my time & experiment. Finally, customers were expecting their cars returned to them as quickly as possible.

I'm tempted to go off further on the subject of tuning these "damn" carburetors, but I'll stop here.

your friend on the DTBB

Hey I got that exact same calibrator, er, i mean 3-pound hammer.
I was confident you would be magnanimous about it, George.
Now that you got the time, this old article may be very insightful and bring some new pleasure with your engines, now that you may have the time to play and tune. It helped bring me to new understanding and proficiency with carbs.

Holleys are not rocket science ... matter os fact they are the most versitile carb on the market .. some reading of the Holley Book and some experience is all thats needed . Oh and a lot of patience.

Mix two or three Holleys together and the amount of patience becomes compounded to the power of a number.
Speaking of that I haven't seen any 2/4, 3/2 Panteras. Are there any around?

We have a guy here in Denver who has a dual carb 427. It has a lot of grunt.

Captain - Is he bringing the car to Automezzi XVI on June 11th? I would like to see the beast.