The spark plugs should be removed and examined periodically.
When disconnecting the HT leads from the plugs, grasp the moulded cap and pull it off the plug. Do not pull on the plug lead itself otherwise the core inside the lead may be damaged or broken.
Blow or brush any dirt away from around the plug base before removing the plug from the cylinder head.
Inspect the condition of the insulator tip and plug electrode as this can give a good indication as to the general state of the engine.
Typical examples of spark plug conditions are shown in , and should be interpreted as follows: Normal (Fig.1) Ideally the plugs should look like the condition shown in this photograph. The colour of the electrodes should appear greyish-brown or tan-coloured. White to yellow deposits usually mean that the car has been used for long periods at high, constant speeds. Provided that the sparking plugs have not covered a large mileage they can be cleaned, re-set and refitted.
Heavy Deposits (Fig.2) The sparking plug in this condition will probably look worse than it is. Heavy deposits could mean worn valve guides.
When deposits have been cleaned off the sparking plug should be fit to use again providing it is not worn.
Carbon Fouled (Fig.3) This is identified by dry, fluffy deposits which result from incomplete combustion. Too rich an air/fuel mixture or faulty action of the automatic choke can cause incomplete burning.
The mixture being too rich can often be traced to a dirty or blocked air cleaner.
Defective contact breaker points or high tension cables can reduce voltage supplied to the sparking plug and cause misfiring. If fouling is evident in only one of two cylinders, sticking valves may be the problem. Excessive idling, slow speeds or stop/start driving can also keep plug temperatures so low that normal combustion deposits are not burned off.
Oil Fouled (Fig.4) These are identified by black wet sludge deposits and is traceable to oil entering the combustion chamber either past the pistons and bores or through the valve guides. Hotter sparking plugs may cure the problem temporarily, but in severe cases an engine overhaul is called for.
Overheated (Fig.5) Sparking plugs are usually identified by a white or blistered insulator nose and badly eroded electrodes. The engine overheating or improper ignition timing could be responsible for this problem. If only a couple of sparking plugs are affected the cause may be uneven distribution of the coolant. Abnormal fast driving for sustained periods can also cause high temperatures in the combustion chambers and, in these circumstances, colder sparking plugs should be used.
Adjusting Spark plugs which are in good condition and with low mileage can be cleaned, preferably with a proper sandblast cleaner.
Clean the electrode surfaces and file them flat with a points file.
Check the electrode gap with a gap setting gauge or feeler gauges. The gap should be 0.025 in (0.65 mm).
If necessary, adjust the gap by bending the outer electrode - NEVER attempt to bend the central electrode, otherwise the ceramic insulator may be cracked or broken.
When fitting new spark plugs, the electrode gaps should be checked before installing them in the engine. Ensure the replacement plugs are of the correct grade - see Service Data for specified types.
Great care should be taken to avoid overtightening the plug in the cylinder head. They should be tightened a maximum of 1/4 turn (90') past finger tight.