Low engine vacuum...

[msbaugh]

At idle, the engine vacuum isn't even above 10" hg, it sits below and it isn't very steady either... I checked for vacuum leaks and couldn't find anything by spraying carb cleaner.  The cam isn't to radical either from what I can tell, but I didn't build the motor, I don't even know what's inside.  It runs great, and compression test turned up really good last time I did it.  170-180 psi on every cylinder. 
What should i think of this...
Do I have an ignition problem that can be solved with advancing the timing?
Would fine tuning the idle mixture screws get it up any higher?
Maybe if it's a valve I should take a look under the valve covers and do a lash adjustment or check for broken springs or something.  It runs fine, but nonetheless I'm nervous about the condition now

I'll post a video so it's clear

[msbaugh]

Low Engine Vacuum

[PlumCraZRT]

At idle, the engine vacuum isn't even above 10" hg, it sits below and it isn't very steady either... I checked for vacuum leaks and couldn't find anything by spraying carb cleaner.  The cam isn't to radical either from what I can tell, but I didn't build the motor, I don't even know what's inside.  It runs great, and compression test turned up really good last time I did it.  170-180 psi on every cylinder. 
What should i think of this...
Do I have an ignition problem that can be solved with advancing the timing?
Would fine tuning the idle mixture screws get it up any higher?
Maybe if it's a valve I should take a look under the valve covers and do a lash adjustment or check for broken springs or something.  It runs fine, but nonetheless I'm nervous about the condition now

I'll post a video so it's clear

The simple question first... are you sure your vacuum gauge is correct?

Advancing the timing could help, but what's probably the most helpful is overall timing.  Remove the vacuum advance tube and plug the vacuum port on the carb.  Rev the engine to 3k+ rpm (so all the mechanical advance is in) and set the timing to 36-38 deg.  You may consider leaving the vacuum advance off (or use it if you're not having pinging problems).

Once the timing is set, play with your mixture screws and see if the vacuum improves.  Use the screws to adjust to the maximum vacuum setting.

[Strawdawg]

What initial timing are you running?  If it has a long duration cam, it will probably make its best vacuum with at least 16 degs of timing for initial.  If you are not running in that vincinity, then bump it up and see what happens.

If that  makes a major improvement, then you will have to follow the above advice and check the total timing, and then make any required changes to the mechanical  advance so that you can run 16 initial with the total in the 36-38 deg range as suggested above.

You did not say if the above was taken in gear if the car is an automatic or in park/neutral.  If a stick, I am assuming that is in neutral or clutch in.  If it is an auto, what stall converter are you running?

It would really be nice to know the cam duration to be able to make some assumption.

Adusting the mix adjust screws can make a little difference, but seldom a big number in most cases.

[msbaugh]

The initial timing is set at 12* BTDC if I remember correctly, I lost my timing light so I'll get another one and check it tomorrow to be sure.  Unfortunately I don't know the cam specs... It is a 727 trans, I put it in drive with the e-brake on when I checked the vacuum.  Also, It's a 2500 stall convertor. 
I haven't vacuum tuned the 4 corner idle yet, I'll do that tomorrow as well as check the timing again.
Also it has those weird iridium plugs in it, maybe I should check them as well or swap them for some regular copper ones, I don't understand heat ranges so I wouldn't know what kind to buy
I'm just trying to narrow down the vacuum issue because I don't know if it's a major mechanical problem or not

[Chryco Psycho]

I have found it helps to get the timing into the 16-20* range , you do not want to overadvance with RPM though & may have to weld the advance slots shorter on the mech advance & do not use vacuum . It is not uncommon to be under 10" of vacuum at idle  , many of the engine I have tuned have had to have 3-4" power valves just to keep them closed at idle , usually when you are under 10" you needf to start looking at smaller bleeds for the idle circuit & holes drilled into the throttle plates to allow enough air the keep the engine running while still covering the idle transfer slots to get a decent idle .

[TinCuda]

 

[PlumCraZRT]

I have found it helps to get the timing into the 16-20* range , you do not want to overadvance with RPM though & may have to weld the advance slots shorter on the mech advance & do not use vacuum .

I suggested tuning to 36 deg total because of this.  You can look into the kit from FBO:  http://www.4secondsflat.com/Ignition.html

J685K Limiter Kit
All Mopar Electronic Distributors

This kit has a drop in disc that will limits total Mechanical Advance to 18-14-10 or zero degrees.
Includes a spring kit, instructions and the Tuning Guide
$39.00

[msbaugh]

Well... I tuned it with a vacuum gage this morning, in drive the highest I could get it to was 7" hg vacuum idling at around 750 rpm. In park it gets up to about 10", the manifold vacuum jumps and acts fine under throttle like it's supposed to.
I also hooked the timing light up it was at 12* initial timing just what I thought, I got it to 3000rpm and it was at about 32* I bumped it up to about 36* and 15* initial timing, I don't want to go much more than that. The vacuum gage did go up very little, but not much. At idle it pretty much stays under 10" Hg no matter what

I'm coming to the conclusion that it's just the cam profile doing it. Who knows I don't think there's much else I can do unless I can figure out a way to search for a vacuum leak that may not exist.

Should I even be concerned?

[Chryco Psycho]

sounds right to me , more aggressive/ long duration cams will drop the vacuum down .
What carb are you using & is it tuned for low vacuum  ?

[msbaugh]

Well it's a 750 Holley double pumper, mechanical secondaries, no choke, original jets sizes that came in it and four corner idle. I just turn all 4 screws 1-1/2 turns out from all the way in, then adjust them all the same until vacuum is the highest. Also I took it out and it actually seemed like power was way down, maybe it doesn't like the extra advance?

[Strawdawg]

it should not do that.   I have run more than 20 degs advance initial without problem on street cars, and locked advance on race cars.

Do you have both springs on the advance mechanism and the vacuum advance disconnected?

I am assuming a decent cam duration.  If the car is spinning over easily on the starter when cranking, I would assume it has enough.

What is your vacuum with the car in Park so there is no load on the engine?

[high perf mopar]

Well... I tuned it with a vacuum gage this morning, in drive the highest I could get it to was 7" hg vacuum idling at around 750 rpm. In park it gets up to about 10", the manifold vacuum jumps and acts fine under throttle like it's supposed to.
I also hooked the timing light up it was at 12* initial timing just what I thought, I got it to 3000rpm and it was at about 32* I bumped it up to about 36* and 15* initial timing, I don't want to go much more than that. The vacuum gage did go up very little, but not much. At idle it pretty much stays under 10" Hg no matter what

I'm coming to the conclusion that it's just the cam profile doing it. Who knows I don't think there's much else I can do unless I can figure out a way to search for a vacuum leak that may not exist.

Should I even be concerned?

after watching your video just the sound of your cam and watching the vacuum gauge points to the cam..
because thats the same boat im in with a 306 total duration,,262@50 profile..im running 20 intial/34 total,with another 10 vacuum for a total of 44 degrees just to get it to idle without dieing in gear,,vacuum reads appx 10 inches in nuetral,4.75 in gear,,running a 2.5 power valve to keep it from flooding..

my problem IS the wrong cam thats causing the low vacuum...............

[msbaugh]

In park it's around 10" vacuum. I left the vacuum advance on the distributer unplugged. It's a Mopar ignition, I'm thinking a MSD Is in store. That way I can customize it a little easier and have the added capabilities like the rev limiter and the multi spark. As for vacuum, it really hasn't been an issue up to this point. It idles fine, on hot days my fuel pressure drops to zero but i belueve that's just because of vapor lock. I run manual brakes as well so that doesn't effect me.

I was just shocked by how crazy low it was!!

[Strawdawg]

Not knowing the cam, it is really hard to know...might well be that gauge as I have seen some variation at times between gauges.

I am more curious as to why it did not benefit from the extra advance...