Troubleshooting with a Vacuum Gauge

Most Engines Will Have a Normal Gauge Reading of 15 to 21 inches of Vacuum.
Before using Vacuum gauge, engine must be at normal operating temperature. Distributor advance, Must Be Working properly and ignition Timing Set to Specifications. Use Tachometer To Be Certain That Engine Is Running at Specified RPM. Vacuum Gauge must be Connected to intake Manifold.
Following Chart Indicates Possible Malfunction,
of Various, Vacuum Readings
* Average and steady at 15-21 inches
    = Normal
* Extremely low holding steady Reading with indicator holding steady
    = Air leak at intake manifold or carburetor; incorrect timing; under powered boat; fault in hull
* Indicator fluctuates between high and low reading
    = Blown head gasket between 2 adjacent cylinders (check with compression test)
* Indicator fluctuates very slowly, ranging  4 to 5 points
    = Carburetor needs adjustment, spark plug gap too narrow, valves are sticking.
* Indicator fluctuates rapidly at idle, steadies as RPM is increased
   =  Valve guides may be worn
* Indicator continuously fluctuates between low and normal reading  at regular intervals
    =  Burned or leaking valve
* Indicator drops to zero as engine RPM is increased
    = Exhaust system is restricted.


* Indicator holds steady at 12 to 16 drop back to zero and
back to about 21 as throttle is engaged and released
    =  Possible piston ring leak
                     ( check compression )