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Series Land Rover: Common Rich Petrol/Air Mixture Problem - Part I
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Last Updated
12/7/16


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Land Rover Series II, IIA and III (Maintenance and Upgrades Manual)
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The issue of a rich fuel/air mixture has been a topic of concern for a few years now amongst Series Land Rover owners. Various opinions have been expressed as to the possible causes of this problem and I will try to address the most common ones here. A possible solution will also be suggested - certainly worked for me.

The easiest way to discover if your engine is running on an over-rich mixture is to wipe the inside of the exhaust pipe with your finger when the exhaust is cool enough to do so. A rich mixture will be indicated by a dark black sooty deposit on your skin: this is due to the fuel being starved of enough oxygen to completely burn it. What you see in the exhaust is black carbon - this may not show up as black smoke when the engine is running though unless the mixture is very rich. A useful aid to observation though, is to check the exhaust emission against a light coloured background - such as an old sheet placed on the floor beneath the exhaust tailpipe. BUT remember that the exhaust also contains carbon monoxide gas that has no colour, no smell and can be fatal within minutes. So do not test for rich mixture within the confines of a garage, even if the door is open. It should also be realised though that standard Series Land Rover engines take time to warm up and short journeys around town are often not sufficient. So if your vehicle is displaying rich mixture signs within the exhaust tailpipe you need to ask yourself if it could be due to over use of the accelerator with a cold engine - alternatively there is an understandable pressure on Series Land Rover drivers to press the accelerator harder than really necessary simply because the vehicles behind and their impatient drivers belong to a different generation.

One possible cause of the problem is believed to be low quality carburettors that are available cheaply and produced outside of the UK. The carburettor fuel jets need to be extremely accurately manufactured and if this is not the case, then it becomes very difficult to adjust the carburettor to get the optimum fuel/air mixture. Some people have reported that obtaining a genuine carburettor overhaul kit and fitting the new jets from it has cured the problem.

Others have commented that pressure from the fuel pump is too much and can force fuel into the carburettor chamber when it is not required. To investigate this situation I set up a fuel bypass system on a standard Series Land Rover 2286cc petrol engine incorporating a stop valve so the bypass could be switched on or off and performance compared. Part of the circuit is shown in the photo below.

Fuel Bypass
Fuel passes through the in-line filter en route to the carburettor. A T-piece inserted just before the carburettor(hidden in the photo by the top radiator hose), allows excess fuel to return to the pump if the in-line stop tap(yellow) is open.

The by-pass return connection at the fuel pump was via a T-piece with suitable thread adaptor. However, I have not included details of this connection as I feel that this particular arrangement is unsafe long term, due to the difficulty of securing the flexible fuel pipes leading to the T-piece - their continuous vibration could perhaps loosen the T-piece connecting threads. It may however be possible to secure a T-piece and its fuel feeder pipes to the bulkhead, given sufficient access space.

The rubber flexible fuel pipe used in setting up the bypass circuit was of J30 R6 specification - ethanol resistant. Not all fuel pipe has this specification stamped on it and some types can perish rapidly (within weeks) when subjected to 5% ethanol and can be positively dangerous as a result.

I could not notice any obvious difference in engine performance whether the by-pass circuit was active or not. But it could be argued that a longer trial may have lead to a different observation. But there was certainly no dramatic improvement.

This problem of richness in fuel/air mixture is not of long standing and I believe its solution lies elsewhere. I believe that it is related to the ethanol content of petrol. An explanation of why and a practical solution to the problem will be described next month.


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