In the heat of the moment
My mate Leigh Butler is about to fit an original magneto to his Indian 101 Scout and asked for any advice before launching straight in. It’s a bike I built originally and road-tested a few years ago in CB. In the build I fitted a later ‘Edison’ mag; Leigh has finally located the correct ‘Splitdorf’, but is aware he’s got a perfectly-running bike at present and could end up changing that happy situation...
The mag has been rebuilt, but I suggested that my first step would be to make sure it works by testing it hot. Weak magnetos always fail when at engine temperature, so rather than wait until it’s on the bike and miles from home, I always warm a mag in the oven and check it before fitting.
But even then, just getting a spark across a plug gap is a bit of a feeble test; the gap is tiny and you are not working under compression as in the engine. I prefer to see it faultlessly jump a quarter-inch gap to be confident it’s worth fitting, but trying to hold the lead steady at that distance is unreliable – especially when you’re windy about getting a belt of high voltage.
I have an adjustable spark-gap tester to do this, but Leigh came up with his own solution – a piece of stiff card, bent at a right-angle with both leads poked through. He could then stand the mag on the card and, by shifting position, vary the gap from the leads to earth (the mag body). Next thing was how to explain how hot it should be – obviously you don’t want it sizzling, but it should be as hot as it would get fitted to the engine (ie a bit hotter than you can comfortably hold). I suggested he stick it on a domestic radiator on full heat for an hour – it’s winter here, after all! Leigh tried it and the mag still sparked perfectly, so it looks like all will be well.