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Today I was reading one of my many shop manuals for my 1970 MGB and I came across a very good explanation of how to read your spark plugs. This is something I am always trying to do when I check my spark plugs. I hope this will help you too as you try to figure out the many mysteries of the English car engine!
The description is from the MGB 1969-72 Autobook by Kenneth Ball & the Autopress team of Technical Writers, published by Autopress Ltd., Golden Lane, Brighton, BN1 2QJ, England.
THE SPARKING PLUGS:
It is advisable that the sparking plugs are cleaned, adjusted and tested at 6000 mile intervals and renewed at 12,000 mile intervals.
Remove the sparking plugs by loosing each one a couple of turns and then using an air line or tyre pump to blow away loose dust and dirt. The sparking plugs should then, if possible, be fully unscrewed by hand. Store the plugs in their correct order as the type of deposits will give an accurate guide to the conditions in each cylinder.
Examine the firing ends. If the deposits are powdery and range in colour from brown to greet tan, then conditions are normal and satisfactory. If the deposits are white or yellowish then this indicates long periods of constant-speed driving or much low-speed city driving. Provided that the electrodes are not badly worn, cleaning and resetting the sparking plugs is all that is required.
Black wet deposits are caused by oil entering the combustion chamber. Sparking plugs which run hotter may help to alleviate the problem but the only cure is an engine overhaul, or attending to the valve stems, valve guides and valve seals.
Dry fluffy, black deposits are the results of poor combustion. They may be caused by running with too rich a mixture, defective ignition or excessive idling.
Over heated plugs have a white blistered look about the centre electrode and insulator. The electrode may also be badly eroded. This may be caused by poor cooling, incorrectly set ignition, incorrect grade of sparking plug or running at sustained high speeds with heavy loads.
Check the external portion of the insulators, cleaning them with fuel or methylated spirits (denatured alcohol). Renew any plug that has a cracked insulator.
Have the sparking plugs cleaned on an abrasive blasting machine and then pressure tested, after attention to the electrodes. A plug may be considered in good condition if it sparks continuously at a pressure of 100 lb./sq. inch when the gap is .022 inch. Before pressure testing, file the electrodes until they are clean, bright and square and then set them to the gap given. Before refitting the sparking plugs to the engine their gaps must be set to .024 to .026 inch. When adjusting the gap never bend the centre electrode.
Before refitting the sparking plugs, clean their threads with a wire brush. If the threads are still tight, use a well greased tap to clean out the threads in the cylinder head. Failing a tap, use an old sparking plug with a cross-cut down the threads. The sparking plug threads may be greased using nothing but graphite grease. Screw the plug in by hand, using a box spanner for the last few turns only. Tighten the plugs to a torque load of 27 to 30 ft. lb.
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