NORMAL: 17-22 inches of Mercury and steady
LATE IGNITION TIMING OR LOW COMPRESSION: Low (10-15 In. hg.) and steady.
VACUUM LEAK: Very Low (4-9 In. hg.)
NEEDLE FLUCTUATES AS ENGINE SPEED INCREASES: Ignition miss; blown head gasket, burned/leaking valve, weak valve spring.
GRADUAL DROP IN READING AT IDLE: Starts high, say 22 in, then falls lower than 10: Excessive exhaust back pressure.
INTERMITTENT FLUCTUATION AT IDLE: Ignition miss, sticking valve.
DRIFTING NEEDLE Corresponding variation in engine revs: improper idle mixture, minor intake leak, or, if a dual-carb car, improperly synchronized carbs.
HIGH AND STEADY: 22 & higher in. hg: early ignition timing.