READING YOUR SPARK PLUGS
 

 

The appearance of the firing end of a spark plug graphically reflects the condition of an engine, the suitability of the spark plug heat rating, and whether or not the carburetor and ignition timing are properly adjusted.                                                                                   

This pamphlet is intended to assist you in correctly choosing your spark plugs and determining the performance condition of your engine.
Even plugs which present a good appearance, such as those shown in Figures 6~24 can quite often be covered with a lead deposit which causes misfiring.
Wet plug firing ends such as shown in Figures 1~2 are normally attributed to one of the following causes:  (1)  Excessive choking.  (2)  Trouble within the ignition system.  (3)  Oil pumping past worn piston rings and valve guides.
The causes of sooty plugs like those shown in Figures 3~4 and 5 are usually the result of:  (1) A plug with a too high heat rating is being used and the plug firing-end does not reach the self cleaning temperature (above 400~450 degree C.) due to light load conditions.  (2)  Use of a too rich air fuel mixture of richer than 8:1 ~10:1  (3) Trouble in the ignition system.  (4) improperly functioning cooling system resulting in excessive cooling.
The firing end burns illustrated in Figures 25~26 and 27 may be a result of:  (1) Too low heat rating, permitting the plug to exceed the highest limit for optimum operating temperature of over 850 ~ 1000 degrees C due to excessively heavy load operations.  (2)  A too lean air-fuel mixture.  (3)  Ignition timing too over advanced.  (4)  Abnormal combustion such as knocking.  (5)  Cooling system trouble, which causes engine overheating.
Overheating conditions shown in Figures 28 and 29 are due to intense  knocking and pre-ignition following situations identical to those in Figures 25 ~ 26 and 27 where increased temperature of the spark plug firing end results in melting of the electrode.
 

OIL FOULED

OIL FOULED

CARBON FOULED

TOO COLD
       

TOO COLD

COLD OR RICH BUT OK

COLD OR RICH BUT OK

COLD OR RICH BUT OK
       

GOOD

GOOD

GOOD

GOOD
       

REAL GOOD

THE BEST

BEST

BEST
       

BEST

GOOD

GOOD

GOOD
       

KINDA HOT BUT OK

HOT OR LEAN BUT  OK (?)

HOT OR LEAN BUT  OK (?)

HOT OR LEAN BUT  OK (?)
       

TOO HOT OR LEAN PRE-IGNITION RANGE

TOO HOT OR LEAN PRE-IGNITION RANGE

TOO HOT OR LEAN PRE-IGNITION RANGE

TOO HOT OR LEAN PRE-IGNITION RANGE
       

     

TOO HOT OR LEAN PRE-IGNITION RANGE