For those who have or planning to buy vacuum gauge.

Chilton general car care manual

• Gauge reading steady 431.8 to 558.8 mm Hg (8.35 to 10.8 psi) indicates normal engine in good condition.
• Gauge reading low 381 to 508 mm Hg (7.38 to 9.92 psi) but steady indicates late ignition or valve timing, low compression, stuck throttle valve, leaking carburetor or manifold gasket.
• Gauge reading steady but dropping regularly indicates burnt valve or improper valve clearance.
• Gauge reading dropping gradually at idle indicates choked muffler or obstruction in exhaust.
• Gauge reading slowly dropping to zero as engine speeds up indicates choked muffler.
• Gauge reading fluctuating between 381 and 508 mm Hg at idle indicates stuck valve or ignition miss.
• Gauge reading drifting indicates improper carburetor adjustment or minor intake leak at carburetor or manifold.
• Gauge reading fluctuating as engine speed increases indicates weak valve springs, worn valve stem guides.
• Gauge reading vibrating excessively at idle but steady as engine speeds up indicates worn valve guides.
• Gauge reading vibrating excessively at all speeds indicates leaky cylinder head gasket.

Haynes emissions control manual

Testing at various speeds
• Engine starting vacuum should be 25.4 to 101.6 mm Hg (0.5 to 2.0 psi).To test disable ignition (ground wire from coil), hold throttle wide open, crank engine slowly with starting motor.
• Healthy engine at idle should read steady 381and 508 mm Hg.
• Healthy engine at 2000 rpm should read steady 482.6 to 533.4 mm Hg.
• Healthy engine at open throttle should read close to 0 mm Hg.
• Healthy decelerating engine reading should jump to 533.4 to 685.8 mm Hg as open throttle released.

Testing at idle speed
• Low steady reading usually indicates leaking gasket between intake manifold and carburetor or throttle body, leaky vacuum hose, or incorrect camshaft timing.
• Low fluctuating (76.2 to 203.2 mm Hg below normal) reading may indicate intake manifold gasket leak at an intake port or faulty injectors on port-injected engines.
• Regular drops (50.8 to 101.6 mm Hg) in reading at a steady rate indicate probable leaking valves.
• Irregular drops in reading indicate possible sticking valve or ignition misfire.
• Rapid vibration (101.6 mm Hg) in reading combined with exhaust smoke indicates worn valve guides.
• Slight fluctuation (25.4 mm Hg) in reading indicates possible ignition problems.
• Large fluctuation (254 mm Hg) in reading indicates likely weak or dead cylinder or blown head gasket.
• Slow movement through wide range in reading indicates possible clogged PCV system, incorrect idle fuel mixture, or gasket leak between carburetor, throttle body, or intake manifold.

Testing at higher speeds
• Rapid vibration (101.6 mm Hg) in reading at increased engine speed indicates leaking intake manifold gasket or head gasket, weak valve springs, burned valves, or ignition misfire.
• Reading returns slowly to normal and didn't peak above normal (127 mm Hg) after dropping to zero when throttle quickly snapped open (2500 rpm) suspect worn rings.
• Reading returns to normal after long delay when throttle quickly snapped open (2500 rpm) suspect blocked exhaust.