A small amount of penetrating oils should be placed at the base of the spark plug if any difficulty is experienced while removing them. Allowed ample time for penetration. When removing the spark plugs identify each plug with the cylinder from which it was removed so that any faults revealed on examination can be traced back to the cylinder concerned.
Examine both plugs for signs of oil fouling. This will be
indicated by a wet, shiny, black deposit on the central insulator.
This is caused by excessive oil in the combustion chamber and indicates
that the piston rings or cylinder bores are worn . Worn valve guides
can also allow excessive oil into the cylinder head.
Next examine the plugs for signs of petrol (gasoline) fouling. This is indicated by a dry, sooty, black deposit which is usually caused by over-rich carburation, although ignition system defects such as discharged battery, faulty contact breaker, coil or capacitor defects, or a broken or worn out cable may be additional causes. To rectify this type of fault the above mentioned items should be checked with special attention given to the carburation system.
Over-heating of the spark plug electrode is indicated by severely eroded electrodes and a white burned or blistered insulator. This type of fault is usually caused by weak carburation, although plugs which have been operating whilst not being screwed down sufficiently can easily become over-heated due to heat that is normally dissipated through to the cylinder head not having an adequate conduction path. Over-heating is normally symptomised by pre-ignition, short plug life, and "pinking" which can ultimately result in piston crown failure. Unnecessary damage can result from over tightening the plugs and to achieve a good seal between the plug and cylinder head a torque wrench should be used to tighten the plugs.
If you remove the air cleaner or filter from your machine it will affect
the carburation of the machine and hence my adversely affect the grade
of spark plugs used. Normally the type of plugs quoted by the
manufactory are satisfactory for general use of the machine. If the
ignition timing and carburation settings are correct and the plugs have
been correctly fitted, but over-heating occurs, then it is possible that
carburation is being adversely affected by an air leak between the
carburetter, manifold and the cylinder head. This possibility must be
checked thoroughly before taking any further action. When it is certain
that none of the above mentioned faults are the cause of over-heating
then the plug type and grade should be considered.
The spark plugs can be a valuable source of information on the health
of your engine. Inspecting the spark plugs gives the operator the
confidence that his carburetter is set up correctly and that
unnecessary oil is not entering the combustion chamber. Both of
these cause less than peek performance of your engine. Inspect your
spark plugs often, especially when changing brands.
Before re-fitting spark plugs the electrodes should be adjusted to
the correct gap. Also, the threads should be cleaned by means of a
wire brush. A minute amount of graphite grease smeared onto the
threads will prevent any possibility of thread seizure occurring.
But before you crank down on the plug read the tech manual for the
correct torque setting. Approximately 15 to 22 Ft-lbs is sufficient.
When cleaning the spark plugs be careful not the scratch the ceramic
center. This can cause a brake down of it's insulation caractoristics
and cause an alterate route for sparks.
A worn spark plug not only wastes fuel but also overloads the whole ignition system because the increase gap requires higher voltage to initiate the spark. This condition can also affect air pollution.
White deposits have accumulated from excessive amounts of oil in the combustion chamber or through the use of low quality oil. Remove deposits or a hot spot may form.
A plug of the correct grade will bear a light flaky deposit on the outer rim and earth electrode, and these and the base of the insulator will be light chocolate brown color.