There's lots of opinions about how to interpret vacuum gauge readings, and I'm sure all the usual trolls will pick my answers to pieces, but here goes- - - - - -:
Manifold vacuum is a function of the pistons creating a low pressure in the cylinders on the intake stroke, and that lower-than-atmospheric pressure is allowed to get into the intake manifold by the fact that some of the intake valves are open and the throttle plate in the carburetor is closed, at least partway except for full-throttle operation. The closer to closed the throttle can be kept, the higher the vacuum will be. Vacuum is measured in "inches of mercury", literally, the height in inches a column of mercury in a tube can be supported by the vacuum. A complete vacuum, which would be nearly impossible to achieve, would support a column of mercury approximately 30 inches tall. What is actually supporting the column is the pressure of the atmosphere pushing down on the reservoir of mercury the tube is inserted into. No matter how much suction is applied, it is impossible to lift mercury higher than that distance at sea level, and the higher in altitude one goes, the less mercury the atmospheric pressure will support. Very low vacuum is measured in "inches of water" instead of mercury. A perfect vacuum would support approximately 34 FEET of water.
An engine with the right idle mixture and ignition timing, plus good mechanical condition of the rings, valves, and gaskets should have a reasonably steady manifold vacuum around 18 inches of mercury or more. The timing and lift of the camshaft will have a big influence on how high and/or steady the vacuum will get from tune-up adjustments. A single-cylinder miss, whether caused by bad ignition or incorrect fuel, will cause the vacuum to drop every time that cylinder fires, or tries to. The same sort of quick, steady vacuum drop can be caused by a leaky intake valve, or one that's adjusted too tight, letting compression get back into the intake on a regular basis. A steady, but very low vacuum reading can be caused by late cam timing, such as a timing chain or gear installed with the crank a tooth ahead of the cam. A VERY badly worn timing chain can cause similar low vacuum- - - -but I've never seen a gear drive cam get that bad without losing a tooth or two. Sloppy distributor bushings can cause erratic dwell and corresponding ignition timing changes, and that shows up as erratic, unpredictable vacuum fluctuations because of the random effect on ignition from one cylinder to another. An engine that lacks hill-climbing power and has extremely low manifold vacuum with even slight throttle opening under load is exhibiting symptoms of an exhaust restriction, like a plugged-up muffler or a catalytic converter that's clogged with carbon from an oil-burning engine. Obviously, low, erratic vacuum can be caused by badly worn piston rings. This symptom is usually accompanied by lots of blow-by from the crankcase vent system and blue exhaust smoke, as well as excessive oil consumption. In the days before electronic diagnostics, a vacuum gauge, dwell meter, and a compression gauge were all the tools an experienced mechanic needed to troubleshoot an engine. The same techniques work today!
Jerry
