Magneto Advice please

[Nick Turley]

Sadly I suspect the windings Alan.
Try warming the mag in the oven to working temperature and test again. I think you may find a very weak spark.
There is a vent in the BTH that requires a gentle flow of air to cool it. This is often blocked with shellac from previously failed windings.
A lot of mag restorers do not clear this - it is a pesky thing to do.
My recent experience with mags is that to get reliability you need a hotter plug D21 - with D16 they start to miss on the overrun - the mag windings then heat up and it then may fail.
I know there is a safety spark earth in the mag that supposedly protects it - these are not very efficient and are rarely correctly adjusted

Since I have adopted this approach i.e. the D21 plugs mags for me are very very reliable.

I am convinced this is a change needed for modern fuel. I am afraid I don't understand why.

Hope this is of some help to you.

[Bob Culver]

I have no expereince of car magnetos but many sources emphasise that wide or even normal plug gaps shoud be avoided and longer than normal test sparks not drawn. If the safety gap is maladjusted this would be esp so. The Pratt and Whitney double Wasp engine runs/ ran with just .012" gap.

[Robert Leigh]

When I had a failure on a BLIC on our Chummy I replaced the aluminium cover with a piece of wood from the hedge and set the plugs to 10 thou, and managed over 300 miles like that.