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Prewar Workshop: Finetuning your magneto

Prewar Workshop

Last week's article was about roughly setting a magneto for timing. Now I want to let you know how to set a magneto to the correct factory setting, where known.

In most cases, if you have an operating manual with your vehicle (or Google can give you one), this book tells you what the correct timing should be. If you do not have this book, which is quite common on prewars, look for any markings on any of the parts on the car that are visible and able to show timing. Most commonly it will be the flywheel. Other common parts are front pulley or crankshaft distributor gear.

If you want to find your marking, turn cylinder 1 to TDC to the point where both valves are closed (end of compression stroke). At this point, find your marking on the known possible point. It might be necessary to sand some rust or paint away to make it visible. Your marking should be on the ‘early’ side of the part, so if you turn the engine in the rotation direction, you will first see your marking before you reach TDC.

Manuals will often state the marking location as “1 inch before TDC” or a similar description, meaning 1” on the circumference of the flywheel for example.

When you find the marking, mark it clearly with for example a white paint stripe or something similar, and mark the fixed point on the engine to which it corresponds in the same way (mostly the flywheel house casting or a similar part).

This operation requires a timing lamp. The timing lamp has a terminal that attaches to the cyl.1 HT lead. This lamp has a high intensity strobe light that fires when the HT lead it attaches to fires. If you point it at the marking you made earlier and start the engine, you can see exactly if your ignition is advance or retarded in comparison to the original engine marking.

To time correctly, retard the ignition fully with the handle, if your magneto is equipped with manual advance. Then start the car and look for the marking with your timing lamp. It’s correct if the fixed marking is exactly lined up with the rotating marking. Otherwise, you need to set it later or earlier.

If you timing lamp is equipped with pre-ignition setting, you can set a number of degrees on your lamp to line up the markings, and the display tells you how far you’re off on the magneto. If It’s not equipped with pre-ignition, you can just adjust the magneto and try again until you get it just right.

Sometimes, setting the magneto timing can be done easily in any direction. For example with a Simms wheel or when the coupling is just clamped on an axis somewhere. In other cases, you have to change the timing gear in the distribution.

It takes a little fiddling but it’s a fun practice. Getting it perfect ensures easier starting, better combustion, more power and better mileage. If you have any questions about this practice, please let me know via the comments or via This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

Article by Jos van Genugten



#3 2017-05-19 12:39
It is my understanding that modern fuel is more volatile and hence retarding rather than advancing the ignition is often beneficial.
#2 2017-05-17 17:07
Hello Graeme, a good addition to the article. Even though I'm just describing basic instructions, it's very worth to keep in mind this is not the only factor of influence to engine performance.
#1 2017-05-17 05:20
This and last week's articles are most welcome for those of us who tune our old cars. But there can be more to it than following the original timing marks. Today's fuels are less volatile than in the 1920s, and I've found that engines from that era will often stand more advanced ignition than the maker recommends. Also particularly with the Zenith carburettors, a smaller idle jet can improve both performance and economy. Our 1928 Le Zèbre Z-10 was tested on a dynamometer after a recent engine rebuild: ignition was advanced by about 6 degrees, and the idle jet changed from 60 to 50 to give a noticeable increase in power at the wheels. For many years our Dodge 4 has run with ignition advanced by about 8 degrees from the stamping.
It's also worth checking the true valve timing: worn chains or gears, or worn cam lobes, or a previous wrong assembly can inhibit good tuning.

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