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DB18 Misfiring

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Hedgie
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DB18 Misfiring

Post by Hedgie »

I have a 1950 Daimler DB18 which was running fine, but has gradually started to misfire, and is getting worse and worse, on misfiring on more and more cylinders.

My understanding is that this is likely to be caused either by the fuel system (lack of fuel getting through to the carburetor), or by the electrical system.

My money is currently on the electrical system, perhaps the high tension side. Can anybody tell me the best way to test that the coil is working correctly? Is there a widget that I can buy to test this?

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migray
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Re: DB18 Misfiring

Post by migray »

One or more of these can help to check the sparks are getting to the plugs https://www.ebay.co.uk/itm/332257743840 ... Sw44BYifev
Just put between the plug and the HT lead if it flashes brightly when the engine is turned over with the ignition on then that side of things is okay.

Stan Thomas
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Re: DB18 Misfiring

Post by Stan Thomas »

Sorry to contradict your advice Migray, but a static "check for a spark" will not determine the problem.

Without being to technological, the primary of the coil neeeds time to charge up before the points open to induce a H.T voltage in the secondary windings. If the points are too close, the "points break" will be not be sufficient to do this - and if the points gap is too wide, they will not be closed long enough to charge the primary. The time the points are closed is called the "dwell angle" - but for ease of maintenance is converted into a physical measurement - usually 0.015" or "fifteen thou".

New points invariably have a flat face to the heal of the moving contact, which wears to conform to the curvature of the operating cam in the distributor., and it is for this reason wich reduces the points gap - so new points should be reset after a few hundred mile.

However, as a very simple start to all engine running problems consider this:

If the misfire is a stecato shock-like "jerk" it is ignition. If it is more of a wavering loss of power over a number of revolutions it is carburation , but given a DB18 has S.U. carbs I believe which give remarkably little trouble - I suspect it is ignition.

So, start by checking the points, and the capacitor (called a condenser if you were born before 1905) then test the resistance of your plug leads.

If the plug leads are copper cored, they should not give trouble, but carbon-trace leads (the core looks string) will eventually go high resistance and result in mis-firing. Also, suspect any suppressors which have been inserted in the plug leads and indeed the plug caps themselves if they are of the suppressor type.

If the mis-fire develops as the engine gets hot over a period of driving, suspect the coil.

Let us know how you get on.

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migray
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Re: DB18 Misfiring

Post by migray »

Sorry Stan you misunderstand what I am suggesting. The HT testers I refer to will flash when the engine is turned over, using the starter motor, if all is well with the ignition side of things. You can get the same effect by watching the spark from the end of a lead removed from a spark plug but you may get a shock if you are holding the wire near to earth so it is not so convenient. If the tester doesn't flash, or has a weak or inconsistent flash then any changes to the coil or points can be quickly tested. I had a problem with the spark plug cap breaking down when it was warm, finally tracked down using this method.

Stan Thomas
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Re: DB18 Misfiring

Post by Stan Thomas »

Nevertheless, as explained in my previous post, an engine-static spark test will not detect a malfunction in the ignition system, only that no spark is being generated either in total (in which case the engine would run at all) or wholly non-sparking on one cylinder - which would produce a constant rythmetic misfire - which is not what the original post describes.

qantasqf1
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Re: DB18 Misfiring

Post by qantasqf1 »

Certainly sounds like an ignition problem. I’m surprised no one’s mentioned the distributor cap, which may be tracking or its carbon brush is failing.
Steve

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Re: DB18 Misfiring

Post by Brian-H »

Get someone to stand behind the car, or better still drive behind.

If there's black smoke it indicates either running rich (which usually doesn't produce misfire), or weak spark. This is because black smoke on a petrol engine is unburned fuel in the exhaust (basically same for a diesel engine). Weak spark causes a poor burn, hence black smoke because the unburned fuel content burns black on the way out of the exhaust stroke.

If you can, go out with a passenger in each car using their mobile phones to find out a correlation with the black smoke, such as puffs of black smoke, or heavier when the load is heavier (throttle position).

Fitting those sparkrite gizmos isn't a bad idea, though Stan has a point about "no load" testing (I presume that's what he means by "engine-static"). If there was a way to fit a camera in the engine bay so that a passenger could watch the gizmos while you drive ....

List of possible causes of poor spark (in no particular order)
Spark plug(s)
Lead(s)
Distributor cap
Rotor arm
Contact Breaker
Contact Breaker setting
Condenser (still called a condenser Stan, as per the other thread)
Coil
Loose distributor body

EDIT - if it's not spark related then it's lean burning, several reasons for lean burn, including faults with an SU carb, but check the spark first as it's easier to do so

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