How a Magneto Works

I spent a winter tinkering with my Stuart Turner and its Lucas SR1 magneto. The engine has always started okay cold, but never when hot. This is what I found by experiment and off the internet (applies to most magnetos I suspect):-

Points:

Keep them clean and gapped correctly. 10 - 12 thou for the Lucas SR1. Make certain the pivot post is clean and lightly lubricated. Over time it does gum up, so strip, clean and reassemble.

My SR1 does not have a felt pad to lubricate the cam. A very small smear of light grease will suffice.

Condenser (Capacitor):

If the points are burned it is safe to assume the condenser is u/s. Get a new condenser and points set.

A faulty condenser also messes up the ignition timing. Never buy an original condenser (sometimes called New Old Stock or NOS). Those left on the shelves are so old they will have deteriorated and probably no better than the one you're replacing. Get a modern equivalent.

Make sure the LT coil, condenser and points connectors are clean and bright when you re-assemble (don't forget the condenser case to magneto case connection). The lower the resistance here the higher the generated LT voltage. This is then multiplied a zillion times to create the HT volts. Look after the LT and the HT will look after itself.

Coil:

LT side will have a resistance of just a few ohms. The HT side several thousand ohms. If HT coils is in the megohm range it is probably open circuit! The HT will jump the break inside and the coil will still work . . for a while.

Magnetic Rotor:

Leave it alone. Never remove it from the casing or tap it with a hammer, the magnets can lose their magnetism. Clean and lubricate the bearings as best as you can in-situ. Clean out as much of the old grease as you can, repack with fresh, then remove most of it. That will leave about the correct amount.

HT Connection:

The end cover on the Lucas SR1 has a conical spring which is supposed to make contact with a solder button on the HT coil. Check the diameter of the spring as it may be greater than the diameter of the button. That means no physical contact and the HT has to jump the gap to reach the HT cable. You may have to improvise to ensure a good connection. I used a copper rove.

HT cable:

Use solid, or stranded, copper cored HT cable. Silver plated if you can find any. Get rubber or silicon insulated cable, not woven cloth even if it looks 'period'.

p>Do not use modern graphite impregnated cores.

Spark Plug Cap (Boot):

Use a non-resistive, un-suppressed, waterproof cap. Your neighbours may not be happy with fuzzy TV and crackly radio, but you will maximize the HT at your plug.

Spark Plug Gap:

Set 16 thou to 18 thou; no wider.

Magneto Body:

It is essential the magneto has the best possible electrical connection between the casing and its mounting on the engine. Keep the base of the magneto scrupulously clean and free from paint, oil, grease, rust or dirt. A smear of electrically conducting grease can be used sparingly.

The magneto case must be allowed to 'breath'. Why? Because every time the points open there is a little discharge of ionised air inside the magneto. Ionised air conducts electricity, and if it isn't allowed to dissipate it can build up until the HT arcs to the case inside the magneto. So don't block any breather holes!

The biggest NO-NO to remember is this: Never, under any circumstances, spin the rotor without having a spark plug earthed to the casing. If you do, the HT coil cannot discharge and it will, one day, burn out.

The second biggest no-no is don't use your body as the HT return path. Trust me on this one. Upwards of 20,000 volts hurts like you wouldn't believe, though you will have reason to be confident your magneto is working.