Magneto Questions

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Magneto Questions

Post by gelfling » Fri May 31, 2019 9:43 pm

Just getting used to driving with coils on magneto after 10 years of using a True Fire/battery ignition. It runs and pulls quite strong on magneto ( much better than coils on battery), but it idles rougher on mag than battery. Is this normal? Wondering what guage of wire should be used from mag to coil box, or if multi stranded VS solid wire would make a difference. I'm currently using a solid 18ga. wire. This is a pretty short run of wire from the mag post to the firewall mounted coil box on my 1912 roadster. It's probably about 10" in length.
Your thoughts will be appreciated. PS: coils have new hardware and have been set up on a HCCT.

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Re: Magneto Questions

Post by AndreFordT » Sat Jun 01, 2019 3:04 am


First try to change the position of the advance lever to more advance and see what happens. Sometimes when running on magneto you need to set the ignition with more advance due to the position where the magneto produce the right current to generate the spark for the ignition.
If you try to start the engine on Magneto you need to set the lever 3 to 5 notices lower as for a battery start.

Did you test your magneto with a 21w 12v and a volt meter over the lamp.
You should find 8V to 12V ac while running your engine on battery.
If you find less, I think your your magnets are a little weak. You can charge them inside the car.

Good luck

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Steve Jelf
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Re: Magneto Questions

Post by Steve Jelf » Sat Jun 01, 2019 9:26 am

All wires except generator/ammeter/battery (12 gauge) are 16 gauge.

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Re: Magneto Questions

Post by John.Zibell » Sat Jun 01, 2019 10:48 am

Use a multi strand wire from the magneto. Current flows on the surface of wire. The multi strand has more surface area so it will conduct better than solid wire with less resistance.
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Re: Magneto Questions

Post by Norman Kling » Sat Jun 01, 2019 8:59 pm

A couple things:
Coils must be matched. If one or more fires at a higher current than the others, it will be rough because it will fire earlier than the others.

The lowest voltage is produced by the magneto at idle and the highest at higher speeds, so if it is not producing enough voltage to fire each coil, one or more will missfire. It might fire on all 4 at a higher speed when the voltage is higher.

Both these problems could be helped by properly adjusted coils.

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Re: Magneto Questions

Post by Oldav8tor » Sat Jun 01, 2019 9:07 pm

Solid wire is usually a poor choice for automotive use or anytime there is likely to be a lot of vibration.
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