• Richard Branch
    15 August 2016 at 15:47 #52075

    My car is fitted with NGK AB6 ‘s but over the weekend I undertook some Lagonda motoring and suffered with oiled-up plugs. Admittedly this was once, after spending 20 mins in a traffic jam in Farnham however it got me wondering if a different plug would cure this? e.g. would a plug that runs hotter be likely to “burn off” any fouling? Or, is there no cure, it’s an 83 year old engine and one has to accept it? I’d be interested in your experiences.

    Richard Branch
    16 August 2016 at 22:52 #52076

    Bought half a dozen NGK AB6 ‘s I assume they are correct, if there’s an alternative plug that may be less prone to oiling up let me know πŸ˜€

    17 August 2016 at 18:41 #52077

    Many years back I was advised to always tune the carburettors on the rich side and in my experience this will occasionally give sooted plugs when left ticking over or stuck in traffic which can give a misfire but should clear once on the move

    I tried several different heat range plugs on my 2 Litre and found little difference so stuck with the generally used type

    If you found it was actually wet oil fouling then that may be something different..but my guess is it was sooting


    Richard Branch
    17 August 2016 at 20:24 #52078

    Thanks Mark, I will see how I get on with some new NGK AB6 ‘s – as you say it was more heavy soot than oil.

    18 August 2016 at 08:55 #52079

    You may even find 6 is too “hard” with modern fuels for ordinary driving.
    It seems that we can use much softer plugs than previously due to the volatility of modern fuels.
    Try 5s and if it doesnt misfire at the top end you would be ok.
    I have a friend who is using 4s in a 319 BMW (1936) and he drives it very hard indeed ! He says they are fine !
    In my experience NGKs are very good and I would stick to those.

    Richard Branch
    18 August 2016 at 10:16 #52080

    Editing thi spost because I understan what Bill is saying now. My understanding is that the 5 is hotter and the 7 is colder. I would have thought that a colder plug would be more prone to sooting. However I am here to learn so am I right in thinking your suggestion is to use a slightly hotter plug?

    Thanks Rich…

    18 August 2016 at 14:29 #52081

    Hello Richard,

    I used to run on the NGK AB6 plugs and they are great but at slow running they do/will foul up.
    Last year when driving through Spain (hottest weather there in 40 years ? 46 degrees in Toledo) I switched to the NGK 7811/BP6ES plug.
    It?s a modern plug so you need an adapter for the thread which I think is good because once the adapters are in place you are screwing into the adapter and not your 80+ year old cylinder head.

    Anyway, in a nutshell the performance with this plug is slightly better than the AB6 and they don?t fowl up as easily at lower speeds.

    Now that my carbs are correctly tuned (with modern fuel in mind) I have been running on these plugs and they are always a light biscuit brown/whitish colour (apparently whitish is best with modern fuel).

    Give them a try, you will not be disappointed (Green Spark Plug Co for best price) and if you don?t like them (but you will) I?ll take them off your hands.

    Best Regards

    18 August 2016 at 14:34 #52082

    I forgot to mention that I find either the NGK AB6 or the NGK 7811/BP6ES are best when gaped at 18 thou.

    18 August 2016 at 18:19 #52083

    Rich, I tend to only think in terms of “hard” and “soft” so cant be exactly certain about cold and hot. Someone else will put us straight I am sure!
    A hard plug ie a 7, 8 or 9 in NGK terms would be used in my experience on a higher performance engine. For instance when we ran a 2.8 Hart engine on about 12:1 compression ratio we used NGK 8s.
    If your engine is fouling either from too rich mixture or oil at lower engine speeds then I would tend to go to a softer plug – more like 5 or even 4. I use NGK 6 on my 4.5 engine and have been thinking of trying 5s with this horrible modern fuel.
    In my experience the Green Spark Plug Co are not only reasonably priced but also extremely helpful and knowledgeable.

    Richard Branch
    18 August 2016 at 19:13 #52084

    Thanks for all your help everyone. I had already ordered a set of AB6’s but will now also order a set of AB5’s and see how I get on. As you say I’ve been reading about plug gaps and didn’t realise there’s so much to learn. With a magneto you get a weaker spark at slower speeds because the mag isn’t producing as much current so a wider gap tends spark erratically at idle speed. A coil ignition is the opposite because at slow speed it gets longer to build up the current so it tends to miss a spark high speeds because the coil is being asked to discharge more frequently. All good fun! πŸ˜€

    18 August 2016 at 20:30 #52085

    If you google ‘NGK spark plug decoder’ and then look at google images you should find the table that compares heat ranges of NGK plugs with other makes of spark plug.

    For NGK the higher the number the colder the plug, lower the number the hotter the plug

    Hope this helps

    21 August 2016 at 08:43 #52088

    Searching for something else, I just came across a page I copied from one of my brother’s Alvis club magazines, giving a comparison of suitable plugs available in the 1990s. This states “Worn engines will require a plug one stage warmer to burn the oil off the electrodes.”
    Cold to hot for then available 18mm NGK plugs is:
    A8 – A7 – – – AB6 – AB2. So that indicates your choice would be AB2. If you wanted to try a plug between AB6 and AB2, your only choice is the Champion D21.
    Hope this helps. But don’t overlook that that oil indicates that something is getting worn and will need attention in the future. Changing the plugs might eradicate the symptom, but it won’t be a cure!

    Richard Branch
    21 August 2016 at 13:58 #52089

    Thanks Laurence, indeed from my various Googlings I was coming to the conclusion that NGK AB’s 5,4 & 3 don’t exist and I’ve not yet had anyone say they use an AB2. Perhaps it’s too hot?

    As you say I am sure that the engine will need looking at at some time; valve guides & rings would be the obvious wear points but I know from my other cars that if you replace rings you may as well replace the pistons and rebore the cylinders. And if you do that you may as well regrind the crank and fit new bearings. As for the valve guides well that’s new valves & springs too… how much is an engine rebuild? πŸ˜†

    21 August 2016 at 17:26 #52090

    Not necessarily, from that comparison, other manufacturers make hotter plugs than the AB2, and there is only one gap between the AB6 and AB2….and the only plug in that heat range is the Champion one mentioned.

    Difficult, but you really need to establish if it’s rings or valve guides. New rings and a light hone certainly worked for my 205CTi, but metallurgy is better today, and our old engines wear more quickly than their modern counterparts.
    Whatever it doesn’t sound like this is an everyday problem, so I think you can kick the ball down the line a bit. Incidentally, I was reading up on Scintilla magnetos today, and oiling up of plugs can result if you have fitted suppressors (resistors) or modern resistance core leads…all a big no-no with magnetos.


    22 August 2016 at 08:21 #52091

    Even on coil ignition engines I think you will find that suppressed leads or plugs (R in the identity usually) will lower the intensity of the spark and should be avoided. That is unless of course you value your relationships with neighbours or family who enjoy watching television !

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