Vacuum gauge chart
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Normal engine  

Steady gauge 18"-20"  
at all speeds.  
Throttle is  
released and engine  
quickly drops from over 2000 RPM to idle.  
Needle jumps 2" - 5" above normal, then drops to normal without pause or hesitation.

Needle wavers rapidly
between 10"-20" at idle, becomes worse at higher RPM. Weak or broken valve spring causing valve to open slowly. 
Needle wavers irregularly
Needle wavers at idle, fluctuates rapidly in smaller range at higher RPM. Manifold leak at intake port.
Steady low
Less than 2" down from normal.  
Retarded ignition timing.
Drifts at idle,
stabilizes at higher RPM
Burnt valve; combustion chamber leak.
Steady very low

Late valve timing

Wavers irregularly
in one range despite engine speed. 
Unbalanced carburetion; improper spark plug gap; ignition timing; poor valve seating.
Steady Extremely low 

Up to 15" drop 
Warped or cracked intake manifold. Bad carburator-to-manifold gasket.

Vacuum averages lower than normal at idle
needle fluctuates almost 3" on both sides of normal. 
Worn valve guides admitting air, and upsetting carburetion.
Needle rhythmically drops
1"-7" below normal 
Leaking combustion chamber or valve; ignition or plug failure effecting one cylinder 
Needle drops rapidly
but intermittently and then recovers. Valve sticking at times, or won't close properly.
Needle jumps 2"- 5" above normal
upon quick deceleration, but hesitates at a higher pressure before returning to normal. Restricted exhaust system causing back pressure.
Drops to 0" then rises
under full throttle with increased RPM. Dirty air cleaner; Undersized carburetor; restriction in exhaust system.
Needle does not jump much beyond normal when throttle is quickly closed, and engine speed is cut from above 2000RPM to idle. Piston rings may be worn and allowing blow-by into the crankcase.