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Series Land Rover - Fault Diagnosis Exercise(Engine Idling Problem)

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Series Land Rover engine

Series Land Rover engines are well known to be tough and to run seemingly forever, even if poorly tuned. Their poor road performance however, compared to modern vehicles, means that tuning needs to be given regular attention.

This month we are considering the case of a 2.25litre petrol-engined 109in Series Land Rover that shows an uneven idling speed after start up. The engine also has a tendency to stall sometimes when the vehicle comes to a stop, even if the engine is fully warmed up. It is fitted with a Zenith 36IV carburettor and electronic ignition, inline fuel filter and K&N air filter, but is otherwise standard for its 1961 year of manufacture. This same problem can arise with other engines, so reference will be made, where possible, to some of these alternative configurations also.

As with any diagnosis situation for a classic Land Rover, there may be more than one mechanical issue with the vehicle operating simultaneously. It then becomes necessary to group together any observed faults that could have a common cause and then temporarliy ignore the rest. So lets begin by disregarding any observed faults that are not associated with engine running or performance. But will hang on to the observation that the Land Rover seems to be slower ascending hills than it used to be.

Well it has to be either a fuel or electrical issue. With it being electronic ignition, there is no possibility of faulty contact points problem. But it could be faulty ignition leads, spark plug gaps/condition or a failing ignition coil. These are all checked by substitution. Since they are cheap items it's always good to have spares for these critical parts anyway. However, having duly checked these, no problems were found.

So we are now looking at a fuel-related issue. We can illiminate a faulty fuel pump because the engine is receiving fuel and it is the carburettor that regulates the amount delivered to the engine, not the pump itself (though the observed uphill struggling could, in theory, be due to a weak fuel pump). It is certainly worth checking when the aftermarket fuel filter was last inspected or changed. A partially blocked fuel filter could explain the lack of uphill performance - remember the curse of there often being two faults acting simultaneously. With having discovered that the aftermarket paper fuel filter was changed only 200miles back then this should not be a cause of the fuel starvation problem.

So we are now suspecting a carburettor issue - always highly suspect in this type of problem, but we needed to illiminate the other (simpler) solutions first.

Before suspecting an internal carburettor problem it is worth noting that a poorly seated carburettor or ill-fitting inlet manifold can allow excess air to enter and dilute the fuel/air mixture. If the fuel/air mixture is weak enough, then there can be back-firing through the carburettor, but this has not been observed. Nevertheless, a slightly weakened mixture could cause the idling problem without the back-firing so we should continue along the same diagnostic route for the time being. Overtightened carburettor flanges are a frequent cause of flange distortion. To test for a leaking carburettor flange in-situ, you can carefully wipe A LITTLE engine oil around the carburettor flange/inlet manifold junction and watch to see if it is sucked in when the engine is run. The confirmation test to prove this is to remove the carburettor and place it on sheet of fine grade abrasive paper on a thick sheet of glass and rub the carburettor flange over it - there should be an evenly distributed mark across the flange surface. If the surface shows abrasion only in specific areas, then the flange is not perfectly flat and there is a need to rub the flange surface down using the abrasive paper on the glass until flat.

So, with no problem regarding flatness of the carburettor flange, where else could the problem lie? Well Zenith, Solex,Stromberg and SU carburettors all have float chambers which control the amount of fuel entering the engine. So opening up the carburettor and checking the appropriate settings is next on the list. For the Zenith, the distance from the top surface of the top cover gasket (i.e leave it in place) to the bottom of the floats (carburettor inverted) should be 33mm. There is no piston damper as in the case of the SU carburettor so it can't be due to a sticking piston or the damper oil being too thick. The distance is 33mm as expected.

Well that means we have covered all the standard checks and still not found the cause of the problem. Only one thing left to try - something hardly mentioned in any of the repair and maintenance manuals available but something that is regularly carried out in developing countries by roadside mechanics as routine for fuel-related problems. That is cleaning out the carburettor jets. This is normally recommended to be done by using compressed air, but not many Series Land Rover owners have such a luxury. But a second best is to use a standard tyre foot pump that has a flexible hose. So the carburettor is stripped sufficiently to get access to all the jets and the air nozzle held closely against each jet in turn whilst the pump is stroked a few times(by foot or hand). A cut-off plastic biro casing fitted into the flexible hose fastener can help focus the jet stream.

Well the result is that the idling problem is cured when the carburettor is re-fitted. Who knows why the fuel filter didn't stop the tiny piece of debris from reaching the jets, but the filter will be replaced again as there may be more debris on the way.


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