NGK Spark Plug Chart

The information below was taken from a pamphlet given out on NGK Spark Plugs in the late 1970's, it is just as valid today as it was then.


Introduction

The appearance of the firing-end of a spark plug shows how well an engine has been operating, the suitability of the plug heat rating, and whether or not the carburetor and ignition timing are properly adjusted.

The information below is intended to assist you in correctly choosing your spark plugs and deter-mining operating condition of your engine. Even plugs presenting a good appearance, as those shown in Figures 6 - 24, can quite often be covered with lead deposits, which cause misfiring.

Figures 1 - 2 show wet plugs and are normally attributed to one of the following causes:

  • Excessive chocking.
  • Trouble within the ignition system.
  • Oil pumping past warn piston rings and valve guides.

Figures 3, 4, and 5 show the sooty plugs, which are usually the result of:

  • A plug with too high a heat rating being used and the firing-end not reach it's self cleaning temperature of 750 - 840 deg. F. also may be due to too light prop loading conditions.

  • Use of air-fuel mixtures richer than about 8:1 - 10:1
  • Trouble in the ignition system.
  • Excessive cooling usually caused be an improperly functioning cooling system.

The plugs illustrated in Figures 25, 26, and 27 show varying degrees of melting of the electrode and may be the result of:

  • Oil pumping past worn piston rings And valve guides.
  • Plug heat rating too low, permitting the plug to exceed the optimum operating temperature range of 1560 - 1830 deg. F. usually due to operating with an excessively high loaded prop.
  • Air-fuel mixture too lean.
  • Over advanced ignition timing.
  • Abnormal combustion such as knocking.
  • Excessively engine operating temperatures.

Figures 28 and 29 show plugs that have been over heating due to intense knocking and pre-ignition.


Oil Fouled Oil Fouled Carbon Fouled Too Cold
1
Oil Fouled


2
Oil Fouled


3
Carbon Fouled


4
Too Cold


Too Cold OK OK OK
5
Too Cold


6
Cold or Rich
But OK


7
Cold or Rich
But OK


8
Cold or Rich
But OK


OK OK OK OK
9
Good


10
Good


11
Good


12
Good


OK Best Best Best
13
Real Good


14
The Best


15
Best


16
Best


Best OK OK OK
17
Best


18
Good


19
Good


20
Good


Hot but OK Hot but OK Hot but OK Hot but OK
21
Kinda Hot
But OK


22
Hot or Lean
But OK (?)


23
Hot or Lean
But OK (?)


24
Hot or Lean
But OK (?)


Too Hot Too Hot Too Hot Too Hot
25
Too Hot or Lean
Pre-Ignition Range


26
Too Hot or Lean
Pre-Ignition Range


27
Too Hot or Lean
Pre-Ignition Range


28
Too Hot or Lean
Pre-Ignition Range


Too Hot
29
Too Hot or Lean
Pre-Ignition Range