Engine manifold vacuum is affected by carburetor adjustments, valve timing, ignition timing, the condition of the valves, cylinder compression, positive crankcase ventilation system operation, leakage at carburetor, carburetor spacer, or cylinder head gaskets.
17-22 Inches* Normal
Low and steady Late ignition or valve timing, loss of compression due to leakage around the piston rings
Very low Intake manifold, carburetor, spacer, or cylinder head gasket leak
Needle fluctuates steadily as speed increases A parital or complete loss of power in one or more cylinders caused by a leaking valve, cylinder head or intake manifold gasket, ignition system defect, or a weak valve spring
Gradual drop in reading at idle Excessive back pressure in the exhaust system
Intermittent fluctuation An occassional loss of power, perhaps caused by a defective ignition system, or a sticking valve
Needle fluctuates slowly or drifts Improper idle mixture adjustment, wrong carburetor for engine, carburetor spacer or intake manifold gasket leak, or restriction in crankcase ventilation sytem
*Gauge readings of 15 inches or over are considered normal for some six cylinder engines.
High performance and racing engines may have readings considerably lower than 17-22 Inches.
Engine vacuum readings will decrease in higher altitudes.
Engine should be at normal operating temperature before taking reading, with idle adjusted for recommended rpm, and transmission in neutral.
MILEPOSTS Garage | Auto Brevity

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