Spark Plugs and the Series Land Rover:
Petrol doesn't burn well without a healthy spark. If your Series Land Rover is not performing upto scratch then the spark plugs could either be the cause or perhaps indicate what the true cause is. You need to remove the plugs and to do this you risk letting cylinder-damaging particles fall into the combustion chamber. So clean around the plugs carefully BEFORE you remove them.
A healthy spark plug should have both electrodes showing a light brown coloration. If the deposit is dry and black then your carburettor is running on a fuel/air mixture that is too rich. You might even have had a clue that this was the case by noticing a slight smell of unburnt fuel when the engine is idling and a sooty deposit in the tailpipe. If the deposit is wet and black then oil is reaching the plugs and this can come upwards via worn piston rings or downwards via worn valve stem oil seals. If black particles are seen on and around the electrodes then it indicates that the cylinder head and pistons need decarbonising. So you would need to remove the cylinder head in that case. A temporary stop gap measure could be to use spark plugs that run at a higher temperature, but this is not a cure and is only worth a try if your Series Land Rover just cannot be taken off the road to do the necessary work just yet.
Spark plugs can only be effectively cleaned with a specialist plug blaster machine, but some improvement can be noticed by cleaning up the electrode surfaces with emery paper, but make sure none of the emery particles are left inside the plug prior to re-installation; they can scratch the cylinder wall and cause loss of compression.
For a spark to be strong, the gap between the electrodes has to be the correct size. Check with a feeler gauge and set the gaps to 0.025in.
The spark plug shown above is taken from an engine that needs decarbonising - note the small dark deposits on the electrodes and also on the threads. Note the threads should always be wire brushed before re-inserting the plugs into the cylinder head. A torque of 30 lb.ft is normally used but a firm turn on a spark plug T wrench is about right.