1953-1982 Corvette Engine Vacuum Diagnostic Help

Typically we called this the old fart vacuum chart.  Or what does my vacuum gauge tell me.

On a typical engine:

Steady 17-22 hg = normal engine operation.

Low but steady = Late ignition, valve timing or low compression.

Very low = Vacuum leak.

Needle fluctuates as RPM’s increase = ignition miss, blown cylinder head gasket, leaking valve  or weak spring.

Gradual drop at idle = excessive back pressure in exhaust system.

Intermittent fluctuation at idle = Engine miss or sticking valve.

Drifting needle = Improper idle mixture adjustment, carburetor not synchronized, or minor intake leak

High but steady = Early ignition timing.