Not enough vacuum?

-
hula

hula

Well-Known Member
Joined
Jul 19, 2017
Messages
72
Reaction score
22
Location
Norman,Oklahoma
72 Dart 340 has a choppy cam and I don't have the specs. Running 15 inHg vacuum out of my intake. The booster was just rebuilt by dewey boosters so I know it's not a bad booster. Think I need to run a vacuum canister? I hate to add equipment that is not stock.
 
15" is pretty good you shouldn't have a problem
I had a 572/296 cam in my cuda and had no problem with the brakes and I know I didn't have 15" more like 10"
 
I agree with brian6pac,
but choppy cam and 15" vacuum are not usually found on the same engine unless your idle speed is way too fast: which usually points to a bad tune. Or the engine has some mechanical problem

Too late about adding non-stock equipment..... you already stated choppy cam!,lol.
 
You are implying that your brakes need high pedal effort? The booster is not assisting in braking? I agree with the others here. 15" vacuum is plenty to operate the booster. Got 2 vehicles that are running 7" to 10" and the booster works just fine.
 
Ok, so if I should be able to have plenty of boost @15” what else should I look at? Calipers are new and bled.
 
First, you could disconnect the hose to the booster and plug the intake side. See if there is any change in pedal effort.

You did not state what the actual problem is.
High pedal effort but, still stops o.k.?
high pedal effort but, weak braking?
long pedal stroke, short pedal stroke?
 
Pull the hose at the booster and measure the vacuum. Or at least put your thumb over the end. 15" is pretty strong.
 
iScamp said:
First, you could disconnect the hose to the booster and plug the intake side. See if there is any change in pedal effort.

You did not state what the actual problem is.
High pedal effort but, still stops o.k.?
high pedal effort but, weak braking?
long pedal stroke, short pedal stroke?
Click to expand...


Short pedal stroke stops ok but can not lock up with hard pedal pressure. Go 30 mph put in neutral and idle up and will lock up with firm pedal pressure. These brakes should be more sensitive.
 
FWIW, I have manual disc brakes on mine (a Stainless Steel Corp setup), and they work GREAT.
 
You probably are correct that your engine vacuum is too low. BUT, I would confirm this by measuring vacuum at the booster with the car in Drive, foot on the brake.

Where is your vacuum hose connected at the engine?

BigBlockMopar suggests vacuum at the carb. base. I'm thinking of the usual source at the intake plenum.
 
iScamp said:
You probably are correct that your engine vacuum is too low. BUT, I would confirm this by measuring vacuum at the booster with the car in Drive, foot on the brake.

Where is your vacuum hose connected at the engine?

BigBlockMopar suggests vacuum at the carb. base. I'm thinking of the usual source at the intake plenum.
Click to expand...

It’s a 4 speed car and I measured vacuum where the hose comes off the intake. I haven’t checked it through the check valve yet. I wanted to see what vacuum was at the source. I never considered vacuum being different off the intake vs the carb.
 
Probably wouldn't be different from the two sources. Both are below the throttle plates but, the OEM ran it off the intake plenum. I tend to go with that.

Well, 15" at the source should cause you to bump your head against the steering wheel when you apply the brakes hard.
Decelerating in gear and decelerating in Neutral would not make any difference if I had to guess. Actually, decelerating in gear usually causes manifold vacuum to go higher. But, your road test suggests something different going on.

"The booster was just rebuilt by...." Rebuilt and overhauled are some of the most abused words in the English language.
 
P.S. AJ/FormS said: choppy cam and 15" vacuum are not usually found on the same engine....

I agree and am a little perplexed at what you have.
 
iScamp said:
P.S. AJ/FormS said: choppy cam and 15" vacuum are not usually found on the same engine....

I agree and am a little perplexed at what you have.
Click to expand...

With the new information, I have an idea;
Your idle speed is way too high for a 4 speed, OR the engine speed is not returning to idle properly. OR, you are leaving it in too high a gear, while simultaneously braking, OR, you are just unfamiliar with how to operate a manual trans.behind a torquey 340
Here's my thinking;
It stops properly when in neutral with a high hard pedal. Is that correct?
But when still in gear, it does not; but the pedal is still high and hard. Is that correct?

Well then it seems to me that while in gear, the engine is fighting the brakes, and that would be because it is pulling, and the only way that could be is if the idle-speed is too high; and there could be several reasons for that..
Ima thinking the choppy idle is pointing to vacuum leak and someone has compensated for it by cranking in the speed screw.
Ima thinking; fix the idle and your brake issue will go away.

To that end;
Prove the booster is not sucking air and that the check-valve is working. OR just disable it for the following adjustments.
Make sure the PCV is hooked to the base of the carb on the nipple provided for it,
Close your secondaries up tight but not sticking,
disable the Vacuum advance,
and back the initial timing down to 10/12 for starters.
Prove your fuel level is correct and stable.
set your T-port sync,
I'm guessing your idle speed should fall in the range of 650 to 750, and when set to that, you should be able to stall the engine by closing one or both of the mixture screws. Those screws should, when the T-port sync is properly set, fall into the sweet spot at around; 7/8 turn for Holleys; and 2.5 turns for all others.
Then watch the engine smooth out and the vacuum climb to maybe 16. NOW; go looking for a vacuum leak, including from the valley to the underside of the intake ports.
If the idle speed does not fall into the zone of 650 to 750 with the above settings, do not change it with the speed screw....... because that will mess up the T-port sync again!
Instead you will set the idle speed with timing and Idle Air Bypass. However, with your vacuum level, I doubt you will need to get into that.
Just in case you missed it, make sure your PCV is properly plumbed and working; it is your factory idle-air bypass.
In the event that I'm wrong about your idle-speed....... IDK,lol
 
Last edited:
You will NOT need a remote vacuum storage device.
On a high vacuum engine, the booster doesn't care about where it is hooked up... so long as the check valve is working at the booster.
On a engine with marginal idle-vacuum like one with a very-late closing intake, the pulsing in the ONE port runner (often #8) can drive the booster wild. In this case it usually works better if I increase the distance to the intake valve by plumbing the booster directly to the plenum; but NOT to the PCV port nor Tee'd into the PCV circuit. As soon as the engine comes off-idle, the vacuum pops up to a level sufficient to activate the booster. But the check-valve is there to maintain the vacuum for at least one long slow brake application.
In my experience with a manual trans;engine braking will keep the single diaphragm booster charged if you augment it with downshifting...... even with a 292/108 cam. I did have trouble keeping the Dual-Diaphragm charged and eventually swapped it out.
 
Last edited:
AJ

CAN I ASK YOU THIS.
IS IT POSSIBLE IF THE VACUUM BOOSTER IS NO LONGER RUNNING WELL IT WILL MAKE THE ENGINE RUN REAL RICH ?
 
If the diaphragm in the booster is ruptured then you will have a large vacuum leak, this will drop the vacuum at the intake making the PV open up causing more fuel to dump, but with the extra air from the booster it might not be rich just a higher idle RPM.
 
And the easy test is just to clamp the booster line; if the rpm changes, that is proof of a failed or non-closing control valve.
If no change then quit tearing your hair out. The test takes less than 30 seconds, and should be part of test #1 whenever a vacuum leak is suspected

Since you have witnessed your brakes locking up at 30 mph under at least one circumstance,I'm still going with post #16.
No brakes can deal with an engine still putting power into the driveshaft. And you said it yourself, in post #8, it stops OK, locking up the tires at 30mph, in neutral, with the engine idled up to evacuate the chamber.
However, since it's not a 4-wheel drive, you should still be able to lock the front wheels; it just won't slow the car down as fast, with the engine still powering the driveshaft.

The booster once evacuated,should store enough vacuum for at least one long slow stop with the driveshaft not powered up. If it doesn't, it's time to test the check-valve. You can do this simply by removing it and sucking or blowing thru it; it's a one-way valve.
If yours has an extra nipple on it, then you can put a vacuum gauge on it. Then start her up. The vacuum reading here should be about the same as at the manifold. When you shut the engine off, the vacuum should remain. How long IDK, but several minutes is all you need. If it decays right away, then repeat the test, but clamp the vacuum line before shutting it off.This will prove the check-valve is or isn't leaking. If no change, then relax the reservoir retaining bolts and pull the reservoir off the firewall about an eigth of an inch, and repeat the test.This will ensure that the control valve is fully closed. If no change and the vacuum is still rapidly decaying, then check to make sure the pedal is properly parking at the top of it's stroke. If it is,then the booster is faulty.
Here's another test for you;
Plumb a vacuum gauge to the manifold and run it up to the cab where you can see it. Now,with the engine off, pump the pedal several times to equalize the booster. Next step on the pedal fairly hard but not two-footing it, and with your eyes on the vacuum gauge, start her up. The vacuum should immediately jump up, and the pedal should fall almost as fast by up to about an inch, indicating that the booster is being evacuated. Once the pedal has stabilized, increase the rpm to increase the vacuum gauge reading and note any further pedal travel. There should not be any.
Finally, return the engine to idle, and shut the engine off. Now press the pedal several times in succession, noting the difference in pedal feel each time. It should take at least three pumps before the booster no longer can be perceived. That indicates the booster is normal, and the check-valve is working.
 
Last edited:
BTW the booster has nothing to do with your brakes ability to stop the car. It only reduces the pedal effort to do so. Think of the booster as an invisible assistant pushing the pedal alongside you; he's just making it easier for you, by multiplying your effort.
 
Last edited:
You must log in or register to reply here.
-

Back
Top