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Zenith 36IV Carburettor Problems - Part II

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The September 2012 Australian homepage focussed upon Series Land Rover Zenith carburettor problems. Items dealt with were: adjusting the float height; adjusting the fuel mixture and checking the accelerator spindle.

This month, the UK homepage is focussing upon further aspects of problems with the Zenith carburettor fitted as standard, or by choice, to many Series Land Rovers and how they may be resolved. The two homepages together provide a more complete commentary on the problems likely to be encountered.

The first thing to note is that all the jets and their respective connecting passages are located in the top half, so simply removing the top of the carburettor is often sufficient to gain access to the problem areas. Once the top is removed then inspect inside the float chamber for sediment and, if found, then you need to replace any additional fuel filters you have fitted. If you have not fitted an additional paper filter then it would be good policy to fit one between the pump and carburettor because the ethanol content of fuel absorbs water and tends to corrode the lining of old fuel tanks more rapidly leading to sediment in the fuel..

If running a rich mixture (black exhaust) has been a problem, then whilst the upper body is removed carefully turn out the volume control screw in the base of the carburettor, counting how many turns are required to remove it. Inspect the pointed end for a wear ring and if present, then probably the screw has been over tightened in the past and it now needs replacing. Unfortunately there is more than one size of volume control screw and repair kits only come with one of them. The volume control screws for the tamper-proof version (surrounded by a plastic sheath) is different from the general one (spring loaded fixing). It may be useful to note the model number of the carburettor on the aluminium tag fixed to a top fixing screw (if still present) when ordering a repair kit. A worn volume control screw can cause erratic slow running and stalling on acceleration. Refit or renew the volume control screw according to how many turns you counted previously. The part number for the locking type volume control screw is 90601848 and for the spring type volume control screw it is AEU1980. As with many replacement parts for Series Land Rovers, obtaining OEM parts is always recommended if you can obtain them.

After removing the float assembly check carefully the point of the needle valve and its seating The needle valve point should not show signs of wear or ridging and the seating should not have any particles embedded in it. If the needle valve cannot seat properly then you will have difficulty starting the engine whilst it is hot. This is because excess fuel will leak into the float chamber and cause an over-rich fuel/air mixture.

The emulsion block can be separated from the main body by the removal of two screws. Care is needed when lifting off the emulsion block as the accelerator pump (a cylinder surrounded by a spring) may drop out. The job of the accelerator pump is to provide smooth acceleration, so inspect this for wear that may indicate that it is sticking. A new accelerator pump (part No. 601856) is not included in most Zenith rebuild/overhaul kits.

Blowing out all the jets in situ and all the connecting passages can often solve poor performance that is carburettor related. Standard references stipulate the use of a compressed air line but if one of these is not available then I have achieved satisfactory results with a foot pump - just ensure the closest nozzle-fit you can, as the pressure is not so high. What you must NOT do is poke things into the jets to try to free a blockage as the surface of the jets is important as well as the size because it affects flow rate.

There is a ball valve below the accelerator pump that is held in position by a spring. Don't remove this ball to clean it, just blow air at it and any foreign material around it should be removed. Carburettor cleaner may help clear stubborn particles. Pay particular attention to blowing air through the passages that end in the air intake part of the carburettor. These passages are tiny so inspect for them carefully.

On top of the carburettor are three screws arranged triangularly. These give access to the fuel economy device - a much required asset for a Series Land Rover!. So if overly excessive fuel consumption was a problem then this could be the cause. `Under the cover is a flexible diaphragm that may be prone to damage with ethanol enriched fuel, so check for loss of flexibility or tears and replace if necessary from a repair kit. Ensure the spring operating the diaphragm is seated correctly when re-fitting. Fitting a new spring and two new gaskets is standard practise. Tighten the three screws evenly so as not to compromise the important airtight seal.

A common problem over time with the Zenith 36IV carburettor is the warping of the body components due to heat from the exhaust manifold. Symptoms of this are a rich mixture at idling speed that cannot be corrected by adjustment. There can also be hesitation on acceleration from rest unless the choke is engaged. The solution is to dismantle the carburettor and sand down the three mating surfaces until flat using 180grade or similar wet/dry abrasive paper. A perfectly flat sanding surface is required and it is standard practise to use a sheet of glass (even a glass mirror) for this purpose. Sand the surfaces using a circular motion and inspect the progress frequently. You are aiming at an evenly bright coloration to the mating surface.

emulsion block
This photo shows the detached zenith emulsion block resting on a flat glass mirror. A 0.002in feeler gauge can slide under the emulsion block up to the point shown - illustrating the degree of block distortion.
See larger image

zenith emulsion block
The darker area shows a depression that has not yet been sufficiently removed by sanding.
See larger image

Zenith carb cover
In this case, a light sanding shows up ripples in the surface(arrowed) that need to be removed by further sanding. Be wary of not using too coarse an abrasive paper as this can also lead to surface rippling.
See larger image.

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