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PreWar workshop: Timing a magneto

Prewar Workshop



Today's article is about a thing I find on many cars: wrong or slightly wrong timing on the magneto (or battery ignition, but I mostly fettle with magnetos so I will concentrate on that).

I always find that if a car is not running correctly, it’s best to first look at....

the ignition for correct timing and adjustment because these are the simplest problems that often give fast results. I want to explain how to simply adjust the magneto to the more-or-less correct timing (enough to run a car) and how to fine-tune it to perfect adjustment. First you need to know a few basic principles:

1. Never turn a magneto without having a spark gap on the high-tension leads, or a spark bridge in general on the magneto. When the magneto is turned, it induces a high voltage. This voltage needs to be able to arc somewhere. Many magnetos have spark bridges. This is an internal parts with a large enough gap to the distributor that it can arc there, but won’t arc there when a spark plug (with a smaller gap) is attached.

If it’s not equipped with a spark bridge, then always have HT leads and a spark plug attached. Otherwise, the arc wants to jump the windings of the magneto and this will damage the winding insulation, causing premature failure.

2. Make sure your spark plugs have a gap of about 0,4mm and the breaker points have about the same. It’s not a very critical dimension if your magneto is in top condition, but the weaker it gets, the harder it becomes for the spark to jump larger gaps.

3. If the plug sparks at atmospherical pressure (outside the cylinder) this doesn’t mean it sparks under full cylinder pressure. I always test under 8 bar pressure in a testing device, which shows much better if it sparks correctly.

Having all the basic settings correct, and knowing the magneto fires correctly, the next step is engine timing. First thing is making sure that you know the firing order. The easiest way to find this out is looking at the valves. Starting on cylinder 1, look at the moment where the exhaust closes and inlet opens (both valves move at the same time). Then look which cylinder does this next and so forth. On a 4 cylinder, this is 1-3-4-2 most of the times, but not always.

Next, set the engine to the cylinder 1 firing moment. This is approximately 1 full rotation of the crankhandle (when equipped) away from the moment both valves move. Most engines will have a marking on the flywheel or another place on the crankshaft (front pulley for example) that marks this exact point. If there is no marking, I set the crankshaft at exactly TDC with both valves closed and mark that point on the flywheel or the pulley. If there is no spark advance on your magneto, you want to be a bit early before TDC not exactly at the point.

Now you set the magneto, still on the workbench, to point to the cylinder that has the no.1 HT lead, and in the rotation direction of the magneto, you add the other HT leads in the firing order. (watch out, the distributor turns in opposite direction to the magneto drive because of the 1:2 gear reduction). (photo 2)

Last thing you do is fully retard the magneto if it’s equipped with spark advance (if not equipped there is nothing to set). And slowly turn the magneto to the point where the rotor still points to the no.1 cylinder and the contact points JUST start opening. This can be done by eyesight, with an electronic aid or simply with a thin piece of paper as feeler gauge (cigarette paper for example). You will feel a little spring tension at this point, that is normal. {afbeelding contactpunten}

I always mark this point at the front drive of the magneto with a marker, because that’s much easier to see when mounting the magneto.

Now make sure the marking stays in that place, and if you have the engine in its correct position, you can mount the magneto like this. Sometimes, you can adjust the magneto in any way possible, for example with a Simms-wheel or with a clamp on the engine shaft. Sometimes a gear in the distributor has to be turned to adjust the magneto position (in my experience mostly found on American cars).

Once you can line up the magneto on its point and the engine on its point and you fit the HT leads, the engine should start easily. If you have spark advance, you can now advance it to the point it runs best at speed.

Next week, I want to talk about fine-tuning this method and adjusting it with a timing lamp.

If you have any questions, please add to the comments or e-mail me at  This e-mail address is being protected from spambots. You need JavaScript enabled to view it ." data-mce-href="mailto: This e-mail address is being protected from spambots. You need JavaScript enabled to view it ."> This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

Article by Jos van Genugten



#7 Iain 2017-05-12 10:21
Ah Jak...nothing gets passed you! And you are right of course!
#6 Jak Guyomar 2017-05-12 07:41
Good luck Mark Farrall timing your Napier.
I agree that it is beaut information & may The PreWar Workshop keep up the good work.
From a fellow Napier owner. jak.
#5 Jos 2017-05-11 06:28
Thanks for the comments. I had these pictures in my archive and hoped nobody would notice. Anyway it doesn't matter in the operation. Always turn the magneto in it's operation direction and turn it very slowly on the workbench so it doesn't generate a spark. The point where it's timed correctly can be seen and felt as described.
#4 Mark farrall 2017-05-10 19:56
Great stuff, just as I am about to time my 1911 Napier that is anti clock wise, I agreed you picture and direction is wrong :-/ good reading thanks
#3 John Hughes 2017-05-10 13:36
"DYKE'S ENCYCLOPAEDIA" (The well-known and readily available period publication), explains it all in very simple terms.
#2 Tony Hillyard 2017-05-10 07:35
I think a lot of people will print this article off for their files of "useful information" Jos. Many thanks.
#1 Jak Guyomar 2017-05-10 01:59
G'day----There always is some clever Charlie that will want to enlighten you!!
The direction of rotation that you show is wrong for that set of points. The moving point should have the "striker block" trailing. ie the magy shown should turn the opposite direction to what is shown.
Just an observation.

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