Series Land Rover - Engine Backfires
Whether the engine is petrol or diesel, the common reasons for the engine to backfire in a Series Land Rover are essentially the same. Basically, a backfire is caused when the fuel/air mixture is ignited outside of the combustion chamber. With a misfire, either some of the combustion takes place in the exhaust manifold or in the inlet manifold. Either way there will be a loud bang, and for a diesel engine especially, the neighbours are going to know about it! Backfires most often occur on engine start-up, but the longer it persists then the more serious the problem is and the more damage that can be caused as a result.
Fuel/air mixture causes
A too rich mixture can ignite too early and burn whilst the inlet valve is not completely closed, causing a backfire into the carburettor and air filter. A too weak mixture ignites too late and burns whilst the exhaust valve is starting to open. So the location of the sound effects can give a clue as to nature of the fuel mixture (it may not the real cause of the problem though).
Fuel/air mixtures can be incorrect due to partially blocked air filters (rich mixture); weak fuel pumps or air leaks in the fuel system (weak mixture); a poorly fitting inlet/exhaust manifold can allow air in to weaken the mixture, as can a loose joint in the inlet manifold; in petrol engines, carburettor faults can cause either rich or weak mixtures.
Valve timing causes
If valves open or close at the wrong times then ignition occurs in the wrong places. Cause of faulty valve timing can be incorrectly set valve clearances (will hear a clattering sound from the rocker cover where the valve settings are wrong). Other causes are worn or damaged valve springs or a worn timing chain. If the timing chain is badly worn (or the chain tensioner ceased up) then you may be able to hear a rattling sound from the front cover of the engine.
Electrical causes(petrol engines)
If the timing of the spark in a petrol engine is not just right, it could cause a back fire. The correct setting of the ignition timing becomes more critical as the speed of the engine increases; the faster the engine revs then the more frequent the backfires will become. The ignition timing can be out due to the looseness of the distributor having caused it to rotate a little; the contact points may be damaged or the gap incorrect.
Finally, it is not unknown for the spark plug leads to be replaced occasionally in the wrong order after having removed them.