AS above check for vacuum leaks ,but is sounds to me that its not fluctuating so I am saying late timing. Advance the dissy with the vacuum line blocked till it just reaches the highest reading then back it off 1 or 2" of vacuum and you'll be damn close [re connect vacuum line]. Check under load it make sure you don't hear any detonation. Your reading will depend on your engine condition too..  



"Came too close to dying to stop living now!"

Advance the Distributor until the Vacuum starts to decrease which means Timing is advanced to far. Turn the Distributor to retard the Timing until Vacuum starts to decrease. Set the Distributor halfway between the 2 points where Vacuum starts to decrease. Check by driving.  If You get a knock from pre-gnition back timing off a little at a Time until Knock stops. Old Time Mechanics used this Method instead of depending on Timing Light.

  

On the face of the vacuum gage is a notation that states In the range you reads !4-15 LATE IGNITION TIMING. This means you need to ADVANCE your timing to the HIGHEST reading.

I like to use the B-12 carb clean with the red straw to focus the spray when testing for vacuum leaks. If I am at the shop, I have a stethoscope with the probe remove and a section of vacuum tubing attached. It is really accurate and you can find the smallest leak with it. Is your reading steady or does it drift? Drifting vacuum reading is generally carb. If the needle is moving quickly, like a vibration it's more likely valves. Just generally low but steady could be rings or timing.

I'm with Charlie, until you get the carb and timing somewhat close, the vacumn reading doesn't mean much. The only thing is if was fluttering, that can be valves. If it was steady, you've got a starting point.

If the motor has a bigger cam in it, a low reading could also be that.

miker
55 bird, 32 cabrio F code
Kent, WA
Tucson, AZ

If the carburetor is in question, then your vacuum reading is meaningless.

First, make sure all intake and carb bolts are tight, all vacuum ports are plugged, all vacuum fittings are tight and all vacuum hoses are in good condition.  If that doesn't change anything, you can use some starting fluid to spray over all gasket areas and fittings.  If a gasket or fitting is bad, the engine will rev momentarily when that area is sprayed.  It is almost impossible to get the underside of the intake, so if all other possibilities have been checked, then the vacuum leak is probably a result of a failed lower section of an intake gasket.

When you do this, be aware that you are spraying a flamable liquid in the vicinity of hot exhaust manifolds, spark plugs, coil, etc.  Have a fire extinguisher handy, just in case.



Lawrenceville, GA

I've been having considerable problems since I upgraded my , carb, distributor and intake, I'm in the process of putting on a new Holley 0-8007 390 cfm carb, the on I have is reman and the vendor is taking it back, I've really had a hard time getting the floats to adjust. Before taking the carb off today I put the vacuum gauge on and let the car warm up, the vacuum reading was at 14 to 15.5 and never got higher than 15.5  , I understand 15 to 20 is normal, can anyone guide me on how I need to find what is causing my vacuum problem?