How to read Spark plug
spark plugs colour and condition tells a lot about what going on inside the combustion camber
such as air to fuel ratio and what is the condition of piston rings and valve seals
air fuel ratio: there are three types of air fuel ratio 1.rich 2.stoich 3.lean
stoich: for a gasoline engine air fuel ratio 14.7:1 is consider stoich. it have 14.7 parts of air for every 1 part of fuel and when it is burned inside the engine all the fuel is burned along with all the air and no excess air left.
we can tell that if the engine air fuel ratio is stoich if the spark plug is brown and light gray colour
spark plug brown tip and ceramic insulator brown color indicates that the mixture is stoich
rich: air fuel ratio is considered rich if it is 12:1 at this air fuel ratio spark plug colour is little black. but not too much black like show in the picture below. a spark plug with too much black colour will consider a carbon foul spark plug. when this happens you will need to lower the fuel ratio if the problem still persists then try changing spark plug.
spark plug Black tip indicates that the mixture is too rich or carbon fouled
Lean: air fuel ratio is considered lean if it is 17.6:1 at this air fuel ratio spark plug colour is white. and all the fuel inside combustion camber will but excess air will left. fuel creates a cooling effect inside the combustion chamber and the right amount keeps the air fuel ratio stable . anything above 17.6:1 will consider unstable and cause knocking and detonation. it is recommended to not go for lean air fuel ratio with carburetors. only run lean with E.F.I systems. because they are precise at delivering fuel.
spark plug white tip indicates that the mixture is lean
Broken valve seals and worn piston rings : if piston rings are worn or valve seals are leaking then spark plugs will have oil residue. inspect both of them and replace the worn part