One of the possible causes of poor engine performance is the absense of spark at one or more sparkplugs. For a 4-cylinder standard Series Land Rover engine it is perfectly possible to be able to start and drive the vehicle with one spark plug not functioning at all; though you will be aware that something is wrong.
A problem somewhere in the ignition system is the most likely cause of a classic Series Land Rover engine failing to start. The easiest way of establishing that there is an ignition system fault, is to check for a spark when the outer electrode of one of the sparkplugs is held in contact with a good earth point on the engine and the engine is cranked (ignition ON).
Having established that no spark is present, it becomes necessary to discover where the problem lies. It could be that the sparkplug itself is faulty, but a classic Series Land Rover engine will often start quite easily with just one sparkplug not functioning. So to check if the sparkplug itself is faulty, repeat the spark test with the plug lead only; pull back the rubber insulation and hold the metal plug terminal(with insulated pliers) a sparkplug gap distance from the good earth point used previously. A lack of spark suggests the problem is not with the sparkplug itself.
So the next thing is to check if the low voltage ignition circuit, including the coil, is functioning correctly. The electrics on a Series Land Rover are not complicated and this is easily done by using a screwdriver to flick open the contact points whilst the ignition is switched on.
If a spark is seen then the problem lies somewhere between the distributor cap and the spark plug lead terminal. Check the carbon brush spring in the distributor cap incase it is too weak or the rotor arm is cracked or too small - i.e. the gap for the spark to jump across is too big. Check the inside of the distributor cap carefully for any cracks that could short-circuit the current away from the sparkplug leads. Finally, check for a faulty sparkplug lead by temporarily substituting it for one of the others and doing the spark test described in the previous paragraph.
If a spark is not seen
The problem could be a faulty coil. Take off the middle high tension lead from the distributor and hold it using insulated pliers next to a good clean earth point somewhere on the engine. If you do not see a spark when the engine is cranked with the ignition ON, then either the coil or the middle lead is faulty. Exchange the lead temporarily for one from a spark plug, if possible, and perform the test again. If a spark is now seen, the coil is OK, so replace the complete set of high tension leads, otherwise fit a new ignition coil.
Another homepage (USA) related to sparkplugs
(To see previous homepages visit the Homepage Archives link)