Vacuum Low

[money pit]

I have a 360 with a 727 trans, 3.23 gears. I was using a vacuum gauge to adjust the carb. The vacuum reading was 12 mm Hg. I tried adjusting the mixture screws, no change in the vacuum level. I checked for vacuum leaks around the base of the carb, using carb choke cleaner. No change in the idle. I had the engine rebuilt about 2 yrs ago. I'm not sure what cam is in it.  The builder said he was going to use an RV cam to get more torque. Could the cam be the problem with the low vacuum. I'm measuring the vacuum off the port for vacuum advance for the distributor. If I rev the engine, the vacuum does go up.  Thanks

[Strawdawg]

You should be measuring vacuum on the port that gives manifold vacuum.  This will be located below the throttle blades.  I am not sure which one you are using. 

An RV cam should give you more vacuum in normal cases.  On a Holley, the butterflies need to be set in the correct position for idle.

[money pit]

I have an Edelbrock carb. I measuring the port that Edelbrock said to use for the vacuum advance. There's pretty strong vacuum on this port. When I disconnect it to set the timing the engine really slows down. The other port has no vacuum at idle. Any thoughts?

[Strawdawg]

The port that has vacuum at idle should be the manifold vacuum.  The other port should have very little vacuum until the throttle is opened.

What is your initial timing set to when the hose is disconnected from the carb?  Disconnect the end that goes to your vacuum advance can and plug the hose going back to the carb.

[money pit]

The initial timing is 10 BTDC. I have the port plugged at the carb, when I'm setting the timing. I need to reset the points. The Dwell per the meter is 21. I don't trust the meter so I'm going to use a feeler gauge. I think I need to get a new meter since the RPM reading is also questionable. The meter is not that old. I had a Snap On meter but it sat for so long it didn't work. Couldn't get it repaired. I'll let you know what happens. I'm going to convert to electronic ignition soon.  Thanks

[Bullitt-]

 Appears Manifold vacuum is on divers side.
If you have power brakes try disconnecting & plugging that line & see what happens. 
Then do the same with PCV.   

https://www.thirdgen.org/forums/attachments/tech-general-engine/44932d1051772333-vacuum-line-timing-edelbrock-temp3.jpg

[Cudajason]

The initial timing is 10 BTDC. I have the port plugged at the carb, when I'm setting the timing. I need to reset the points. The Dwell per the meter is 21. I don't trust the meter so I'm going to use a feeler gauge. I think I need to get a new meter since the RPM reading is also questionable. The meter is not that old. I had a Snap On meter but it sat for so long it didn't work. Couldn't get it repaired. I'll let you know what happens. I'm going to convert to electronic ignition soon.  Thanks

The initial timing seems low to me....what happens if you adjust that to say 16 degrees?  How des the car run? Does the vacuum increase

Appears Manifold vacuum is on divers side.
If you have power brakes try disconnecting & plugging that line & see what happens. 
Then do the same with PCV.   

https://www.thirdgen.org/forums/attachments/tech-general-engine/44932d1051772333-vacuum-line-timing-edelbrock-temp3.jpg

I can see this getting into a discussion about timed vs manifold vacuum for vacuum advance, lets not go there yet.

I think you should get the car running properly with no vacuum advance and then add as necessary.

By that I mean you should adjust the initial timing to get the highest possible vacuum.  This may mane slowing the idle speed down or changing the idle mixture a bit, again to get the best vacuume.  I would start there if you are sure there are no leaks.

[chargerdon]

Depending on what cam is in there.....12 isnt actually low...   why you worried about it ?

[DeathProofCuda]

I agree with Cudajason on this one.  This car would probably perform MUCH better with more initial timing. 
Also, if you are running your vacuum advance off of a ported vacuum connection, and your idle drops when you disconnect the vacuum advance, then you have the throttle blades of the carb set open too far.  Your vacuum advance should not be working at idle.  Instead, get the added advance at idle that you are currently getting from the vacuum advance, by increasing increasing your initial advance.

[Strawdawg]

I came back late, but I agree, I would try 16 degs idle with no vacuum advance applied as Jason suggested.

Normally, RV type cams are short duration and make more vacuum.  As we really don't what cam ended up in the car, we are kinda operating on supposition.  Bullitt's suggestions are worth trying

[money pit]

I was using the wrong port for the vacuum advance. I was using the port on the right for the non emission engine (i don't have any of the smog equipment). I reset the timing to about 16 BTDC with the vacuum disconnected.  The vacuum per the vacuum gauge really improved.  Thanks for your help.

[chargerdon]

I was using the wrong port for the vacuum advance. I was using the port on the right for the non emission engine (i don't have any of the smog equipment). I reset the timing to about 16 BTDC with the vacuum disconnected.  The vacuum per the vacuum gauge really improved.  Thanks for your help.

All depends on the cam.   In my 360/408 stroked la engine i have a Lunati Vodoo 703 cam.   With it and my compression ratio (about 9.75) timing it by ear with vacuum disconnected and plugged it seems to like about 20-22 degrees BTDC the best !   

However if i leave it set at that it will be hard to "turn over" when warm...i.e kickback.   So, in order to achieve 20-22 at idle without kickback for best idle and response, i set the vacuum advance to be about 6 degrees (chrysler electronic distributors have vacuum cans that can be adjusted with an Alan wrench), and plug it into the "manifold" side and then set the timing to about 15 static when then gives me the 21-22 with vacuum connected to give me the best idle and performance. 

I then modified my distributor with a restrictor plate set at 18 max...   16+18 gives me 34 total when under pull as the vacuum goes away on a hard pull...remember the old days when the vacuum wipers would stop when going up a steep hill and the throttle more open?  When cruising that 16 + 6 vaccum + 18 mechanical yields about 40 but that is ok under light load.   At this setting i get about 12-14 vacuum when idling and that's more than enough to operate the power brakes and the advance.  There was an article in HOT ROD Magazine a few years ago describing this same technique.