Brightspark Magnetos

At atmospheric pressure, the voltage required to strike a spark across the 0.020" (0.5 mm) gap of a sparking plug is far less than the voltage required at the elevated pressure that the plug experiences in the engine's cylinder. Therefore, taking the sparking plug(s) out of the engine, laying them on the cylinder head and checking whether the magneto will produce a spark at kickstart speeds is not a sufficient test.

Also, many magnetos (especially twin- and multi-cylinder devices) are provided internally with safety gaps, typically of 6.5 to 7.5 mm, and if the magneto can't find an outlet through whatever is or isn't connected to it, it happily sparks the safety gaps instead. However, for magnetos without a safety gap, if the armature is spun with nothing, or with a large gap, connected to the pick-up, it may cause a breakdown of the insulation around the HT winding or of the slip ring. Once the insulation has been broken down once, it often makes it easier for it to be broken down again.

We would therefore strongly advise against ever trying to make a magneto strike a spark across a gap of anything in excess of 7.5 mm in fresh air. With larger gaps, if the magneto is provided with a safety gap, the spark is more likely to jump there where it can't be seen, so that's a waste of time. If not provided with a safety gap, there is a risk of expensive damage.

Our preferred gap for spark testing in fresh air is 6 mm.

If working on your own, holding the end of the HT cable steady 6 mm from the engine is difficult when kicking the starter. If possible, get somebody to help. If nobody is available, then you can make a test plug by removing the earth electrode from an old, but functional, sparking plug. That then produces a gap of about 4 mm between the centre electrode and the screw-threaded bit of the plug body. Not quite as large as ideal, but simple.

If the magneto won't produce a spark across a 6 mm (or 4 mm) gap at leisurely kicking speeds, and if you've satisfied yourself with all the other checks described earlier, then it probably needs a more complete overhaul. The likely culprits are:

• failure of the condenser;

• breakdown of the HT insulation; and/or

• breakdown of the slip-ring insulation.