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Series Land Rover: Common Rich Petrol/Air Mixture Problem - Part II
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In the July homepage of, ways of establishing if a rich petrol/air mixture was present were discussed. Possible causes of this problem were suggested and an experimental carburettor fuel overflow circuit was described. This month a possible solution to this rich petrol/air mixture problem is suggested that may be relevent to many Series Land Rover owners.

Many Series Land Rover owners recommend not just setting the ignition timing to the recommended figure for a particular engine. Arguments for this are often based upon the fact that decades of wear have changed accurately machined tolerances and so ignition timing is best set on a trial-it-and-see basis. The best observed engine performance when the vehicle is road tested is regarded as the timing to settle for. You can always do a timing light check later and established what the timing actually is that you have set, if you so wish.

Within this practical approach to timing may also lie the solution to the rich mixture problem. The reason for this goes as follows:

It is only recently that ethanol has been introduced into petrol in the UK and is often present at the 5% of content level. Ethanol boils at 78C, which is significantly higher than many of the components of petrol (typically ranging in b.p. from 30C to 125C). So basically, modern petrol is a little less volatile than when Series Land Rovers were coming off the production line, with their recommended ignition timing settings of the time.

Ethanol, being less volatile than some petrol components, needs more time to evaporate before it can fully burn. So advancing the ignition timing to make the spark occur a little earlier can provide this extra time needed and illiminate the production of black carbon from incompletely burnt fuel. More importantly however, is the fact that ethanol is more difficult to ignite than some of the hydrocarbon compounds present in petrol and in addition to this, it burns more slowly than some of them also. So there are several reasons why it would be advantageous to have the spark occur earlier in the combustion cycle to allow for the more complete burning of the ethanol-enriched fuel.

The question then becomes; How do we advance the ignition timing and by how much? Well before adjustments are made, it is important to record the existing ignition timing setting. That way you can always return to your original timing easily if necessary. To do this, simply mark the relative positions of the distributor body and its retaining clamp. The best thing to use for this is white liquid paper correction fluid. It is easily seen and is insoluble in oil and water.

To advance the timing, slightly rotate the distrbibutor body in the opposite direction to that in which the rotor arm turns. The photo below shows the upper mark on the distributor body slightly offline to the lower mark. This was achieved by rotating the distributor body clockwise - the rotor arm rotates anti-clockwise.

Distributor advance

If you have an exhaust digital gas tester you can see if there is an improvement in exhaust emissions. Otherwise, the next thing is to wipe any carbon deposits from inside the COLD exhaust tailpipe and test drive it to see if the previously observed problems, such as reduced acceleration, reduced top speed or stalling have improved. Accelerate between 30-50mph(50-80km/h) and listen carefully for engine ‘pinking’- it's a metallic chattering sound. If heard, retard the ignition a little and road test again until no ‘pinking’ is heard.

With the correctly adjusted timing, you should find that, apart from frequent short journeys around town, the exhaust tailpipe should contain much less black powdery carbon deposits.


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