Using a Vacuum Gauge for Engine Diagnosis
by Atwell Haynes
Best way to check vacuum is to connect a tee into the port on the ZS carb (goes to the flame trap, then the EGR and distributor)
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
INTERMITTANT 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 synchronised carbs
HIGH AND STEADY: 22 & higher in. hg: early ignition timing
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