FAILED FIELD

Eddy Hamley’s been at the electrics, too – the dynamo of his 1955 Tiger 100. Having not ridden it for some time, he wasn’t surprised to find it wasn’t charging. Knowing that the copper commutator gets blackened by the carbon brushes, he cleaned it all up and everything seemed OK – until a voltmeter showed it still wasn’t working.

This is a classic case of losing your magnetism – not Eddy, the dynamo. I’ve known dynamos with the commutator plastered in oil, soot, frayed wires, anything and still work – but without magnetism a dynamo cannot produce a charge. The magnetism is provided by voltage in the field coil, which is created by the dynamo itself. You need a tiny magnetism in the dynamo to get the process started, but this can fade away with standing. The polarity of this magnetism can also change and defeat the process. In either case the way to get a dynamo working (if it is sound) is to run a live wire from the battery and poke it in with the wire inserted in the dynamo’s ‘F’ (usually a green wire) terminal. The magnet thus powered up (and repolarised) by the battery, you should get immediate charge. After running the bike for a minute, it should continue charging if you unhook this booster wire; if not, try again for longer – but if it still doesn’t work it may be that the regulator is faulty. The trick worked in Eddy’s case, so that was one problem tackled.