Home Forums General Technical Help Engine cutting out when hot

  • This topic has 8 replies, 4 voices, and was last updated 3 weeks ago by JAN.
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  • #75683
    Denman Ken
    Participant
    Club Member

    My 105E delux model starts easily enough from cold. However, once the engine has warmed up and I come to a stop the engine cuts out. It is virtually impossible to restart when hot and I have to wait for the engine to cool down before it will start again. I have replaced the coil but the problem remains with the replacement coil.Any suggestions please. many thanks Ken

    #75685
    JAN
    Participant

    When it cuts out, is there any fuel in the float chamber? You can check by loosening the blanking screw at the bottom of the float chamber. Some will come out anyway so you need to leave it for five to ten seconds to ensure it isn’t dregs at the bottom.

    #75696
    rwellard
    Participant
    Club Member

    We all need to be very cautious when allowing fuel to flow near a hot engine. there are a number of reasons this may happen, icing in the carb is one, leaks in the induction system that open up when the engine gets hot is another. Good luck

    #75708
    brianw
    Participant
    Club Member

    Other possibles are Condensor that fails when hot. But if the spark is good then …. Mixture way too rich ( carb…. adjusted wrong or leaking economiser diaphragm or stuck needle valve) or Too little fuel ( as JAN suspects , eg Fuel vapourization in pump or carb/ stuck needle valve) or carb blockage.
    Some (not all) Anglia suffer from fuel vapourisation as modern fuel has a lower evap point, other posts in this section have long discussion & advice.
    Checking the colour of the plugs https://www.championautoparts.com/Parts-Matter/automotive-repair-and-maintenance/how-to-read-spark-plugs.html can show if mixture too rich or too weak

    #75741
    JAN
    Participant

    We all need to be very cautious when allowing fuel to flow near a hot engine. there are a number of reasons this may happen, icing in the carb is one, leaks in the induction system that open up when the engine gets hot is another. Good luck

    There is always a need for caution when working with fuel components, which I’ve been doing professionally for over fifty years. You need to note the difference between a continuous leak on a running engine, where the fuel leaking out is a continuous stream augmented and replaced by fuel delivered from the pump on to a very hot manifold, the heat being sustained by exhaust gases passing through it, and a small, finite amount of fuel which will, in any event, cease running once the float chamber has emptied on to a manifold no longer being heated as the engine is not running. In practice, it isn’t necessary to drain the float chamber; once it is established that there is more fuel in there than a few dregs at the bottom the plug is refitted. But if you’re concerned, a small container to catch what comes out will reassure.

    But you need to know whether or not there is fuel in there as a start point.

    #75775
    rwellard
    Participant
    Club Member

    There is always a risk when dismantling fuel systems on a hot engine. when the engine stops it will heat soak, and remain hot for some time. I have spent over 45 years associated with this subject. including 8 years in the R+D section of Zenith Solex. We would always allow 20 minutes before working on the engine.

    #75792
    JAN
    Participant

    Hmm, I assume then that you’ve never worked to a bonus system? Or conducted fault diagnosis in a normal motor repair workshop? Different scenario to R&D.

    The only time I’ve been involved in an under-bonnet fire was, as it were, after the event: a then eighteen month old Capri was brought in after one such fire. One of the lesser known and undesirable habits of Weber carburettors was for the brass tube forming the fuel inlet to come loose and fall out. With the engine running, the lift pump would then spray a large amount of fuel all over the engine, including the ignition system, as happened here. I had to replace various fire damaged under-bonnet components, including the carburettor, and the bonnet had to be replaced too. As an aside, I ‘salvaged’ the old carb and later passed it on the owner of a modified 105E for spares. Instead, and as it was very low mileage, he reattached the tube and used it on his car, to no ill effect that I’m aware of. But I’ve never had a fire happen to me.

    The point is that allowing twenty minutes for things to cool is not a luxury mechanics can afford, but it would also interfere with diagnostic work. Fuel faults can sometimes be restricted entirely to either cold or hot conditions: fuel vaporisation, prevalent with older engines and modern fuel, is one such in the latter category. The fuel starts to boil and bubbles form in the fuel. The effect is similar to air in brake fluid in that the pressure, instead of lifting the fuel and delivering it to the carb, is dissipated in compressing the bubbles. Fuel no longer flows so the engine stops. If you allow twenty minutes for things to cool, you’ve just wasted twenty minutes, because by that time the temperature will have reduced below the Initial Boiling Point, the fuel is again 100% liquid and the pump delivers it to the carb. So you’ve got nowhere.

    In such a situation, you need to know if there is fuel in the carb, and there are a limited number of ways of establishing this. On these Solex carbs, removing the plug briefly will allow the small amount of fuel, and that amount only, to escape, and you can judge by the visible flow rate whether or not the float chamber is full. If it is, the fuel will fall on to the inlet manifold. Unlike the exhaust manifold, this will be warm, below the auto-ignition temperature, the temperature at which the fuel will ignite without an external source of ignition. It will though vaporise, and such a small amount will do so and dissipate before it reaches the exhaust manifold, which will be at or above the auto-ignition temperature.

    Should no or very little fuel come out, that could mean it isn’t being delivered from the pump or the needle valve is stuck closed, unusual in my experience. How do you check? I know of only one way: disconnect the delivery pipe to the carb and give the engine a quick spin on the starter. Fuel spurts out? It’s the needle valve. No fuel spurts out? It’s the pump or blocked fuel lines. Again, the amount which might be delivered is dependent on how long you keep the starter turning, so you give a quick spin only.

    Now it isn’t a case of just blindly spinning the engine over without taking any precautions and hoping for the best. You disable the ignition system, preferably by disconnecting the LT lead at the coil so there are no sparks. Other things? There used to be a magazine many years ago, ‘Popular Motoring’, I think it was called, which had a genius for getting things wrong. Having traction problems in snow? Fit Town and Country tyres. Good advice, except the photo showed them on the rear of a Mini! The one I liked was an article on carburettors and the photo was of a guy leaning over the carb, air filter off and venturi open, and a cigarette dangling from his mouth! Making sure there are no sources of ignition is a necessity, and I hope our members have that much common sense.

    So diagnosing fuel faults means some dismantling, some small loss of fuel, and these might need to be done with the system hot. As I said in a previous post, I have had to do this – in the real world – many, many times. I have never, ever had the need to grow a new pair of eyebrows.

    #75806
    rwellard
    Participant
    Club Member

    I think we both agree that a degree of caution is needed in these circumstances, which is all I tried to say. You are right I did not work in a bonus situation. I did work in fleet maintenance, where a degree of urgency was often required. I have seen a number of fires on test beds. But we were working with prototype parts that did not always behave as expected. No fire is ever a good thing and all possible steps should be taken to avoid it. I would hate to think anybody was involved in an incedent that resulted in injury or damage. I think we have said enough now. I do not disagree with what you have said.

    #75808
    JAN
    Participant

    Good enough. I’d like to hear more f your experiences in R&D though. I did an honours degree course in Automobile Engineering and so there is some overlap between us. Good times in the lab are recalled!

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