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A Tale of Two Magnetos

Rollo Tomkins

Back in June I purchased a non-running Power Pak engine and joined the NACC.  The engine was stripped down, cleaned, and inspected.  The magneto was a Miller, a simple and well made device with aluminium back plate to hold the coil, condenser and points and a brass-cased rotor with two magnets and laminated soft iron segments.

On reassembly, no spark could be coaxed from the aforesaid magneto.  A test with a resistance meter showed the HT coil was US.  A rewound coil was obtained (shudder at cost) and connected up.  With the engine mounted on the testbed (a Black and Decker Workmate, actually) the magneto was given a flick.  A nice fat blue spark across the gap - terrific!  Within minutes the engine was running for the first time since it had been taken off its parent bike and slung in the corner of some dusty shed.

A week or so later I came across an advert for a Mini-Motor engine.  I had to have it!  Those bitten by the bug will recognise the symptoms.  Nothing else mattered, I wanted it, and would not rest until it was in my possession.  The fact that it was 70 miles away made no difference, I would go and collect it!  (It was Friday and the weekend meant a chance to play with my new acquisition.)

The engine was complete except for the decompressor but, as I have come to learn was normal, the clamping gear and engaging arrangement was long gone.  It was interesting to note however that it had an identical Miller magneto to that fitted to the Power Pak.

Once the engine had been stripped, cleaned and reassembled with new seals and bearings, it was ready for a test run on the Workmate - sorry, I mean testbed.  Using the portable engine mechanical starting arrangement (an electric drill with a socket on the magneto retaining nut) the engine was spun at an increasing speed.  It coughed, spluttered, but would not burst into life.  Took the plug out and tested the spark.  A feeble excuse, blue/orange in colour occasionally jumped the gap.  Hmmm.

The Power Pak now installed on a period Rudge bike, stood in the corner.  Five minutes later it was minus its magneto.  Fitted to the Mini-Motor the engine bust into life when a strap wrapped round the roller was given a sharp pull.  Now came the diagnostics.  What was wrong with the Mini-Motor mag?  Fitted to the Power Pak it again only produced an occasional spark.  Coil and condenser were swapped over and proved to be OK.  Finally I swapped the rotors over.  This was it.  For some reason the Mini-Motor rotor was US.  If I put the rotor from the Power Pak mag onto the Mini-Motor mag: perfect spark.

Back to basics.  The rotating magnetic field from the spinning rotor causes a voltage to be generated in the LT part of the ignition coil.  When the points open, the voltage collapses and as the voltage collapses a high voltage is induced in the HT coil which results in the spark.  (I think I've got that about right - apologies to Mr Anderson, my old physics master, if I'm wrong.)

Anyway, I connected a voltmeter across the LT coil on the magneto and using the two rotors in turn tested the output by again spinning the magneto using an electric drill.  From the working rotor I got 3.5 volts, but from the duff rotor, only 1 volt.

Now going back to the electrics lecture above, the two influences on the voltage generated (besides speed of rotation, and number of turns on coil) is the strength of the magnets and the air gap between rotor and coil.  I measured the air gaps.  It was 4 thou on the working rotor and 3 thou on the non working rotor.  I had no way of testing the magnets, but used a screwdriver blade to try and judge how powerful they were.  Was it my imagination, or did the non-working rotor magnets seem weaker?

I went on-line and typed "Magneto Repair" into the Google search engine.  Back came up "Independent Ignition Supplies", Bideford, Tel: 01237 475986, Web address:

A phone call established that yes, they did remagnetise permanent (or not so permanent) magnets and it would cost me £5 + p&p.

By drilling two rivets which held two of the laminated soft iron segments in place, I removed the magnets, popped them in a jiffy bag with a keeper and posted them off - This was Wednesday.  On Friday, returning from work, a package a waited me.  My magnets were back!  What service!

In the garage that evening, the magnets were refitted and the rotor tested.  The spark was that big you could hear the clap of thunder which accompanied it (well almost).

I know it is possible to remagnetise magnets yourself, but you need a DC supply and a big coil.  If anyone knows how to do it, then I'd be pleased to hear from them.  But meanwhile, I am grateful to Independent Ignition Supplies, and other members may want to give them a try if they have magneto problems.

First published, December 2003

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