My mag’s a fag
Ed Hamley promises me this is his last question for this year. Since having a problem with the headlamp on his Triumph 3T, he’s lost his ignition spark. The bike has magneto ignition and he asks if the two problems could be connected. Also, assuming his magneto is faulty, Ed says he has a spare – but he asks how to check it’s correct for the bike and works before fitting. And will changing it over disturb the timing?
First of all, the ignition magneto is completely isolated from the rest of the bike’s wiring – but some Triumphs have a cut-out button in the headlamp nacelle, so although not directly connected with the lighting failure, it’s worth disconnecting the cut-out wire just in case – it’s attached to the contact breaker cover. It’s easier to tell if a magneto is faulty than if it’s perfect. First test is to attach an ohm-meter lead to the brass segment of the slip ring and to the body of the mag. If the HT coil is good, you should get around 4000 ohms – a couple of 1000 up or down is probably OK. Zero resistance or ‘E’ – open circuit – is not. If the mag actually sparks – fat, blue and able to jump a 5mm gap – the best test is heat. Sitting on a domestic radiator will do – it wants to be ‘engine hot’ – not too hot to touch, but uncomfortable to hold. If the spark is still good, it’s worth fitting.
Twin-cylinder mags are broadly all the same; the rotational direction (look for an arrow on the badge) has to be correct, you can work around the rest – but yes, you will need to reset the timing.