PLUGS’N’SPARKS

I’ve just discovered iridium spark plugs and, although not cheap, they provide easier starting on my Velo. I liked it so much, I bought an additional two for my CSR twin.

However, I have heard that these are suppressed and can do long-term damage to a magneto equipped bike and am wondering if this is true. Any thoughts?

James Griffiths, member

Most motorcycle magnetos were not designed to use a suppressor. The suppressor is usually some form of carbon resistor which also increases the voltage the magneto has to generate to get a spark, which can make getting a spark at kick over more difficult and puts a higher voltage stress on the magneto winding at high revs. A recently rewound and re-magnetised magneto should cope, the older ones could struggle.

Stuart Francis, member

I always used platinum plugs in my Nortons in the 1960s with no problems in either magneto or coil ignition. Nowadays I fit iridium plugs in all my (petrol) vehicles to good effect. Once fitted, they last for many thousands of miles without requiring cleaning or adjustment.

Bob Peckham, member

I’ve always known to never use resistor plugs or plug caps on magnetos, although I don’t know the science behind it. When Champions declined in quality decades ago I swapped to NGK, but they’ve gone off in recent years too. I now put Brisk plugs in all my bikes now, road and race.

The one I use in all my bikes is non-resistor so suitable for magneto ignitions, with a non-resistor plug cap, or use a resistor plug cap if you’re on coil / electronic. I doubt you’d have any problem with one in your AJS, but you can always contact Brisk directly.

Odgie

David Angel at F2 Motorcycles stocks Brisk plugs and has quite a bit of info about them on his site at www. f2motorcycles.ltd.uk/brisk_spark_plugs.html Rowena

And I use Brisks almost exclusively now… Frank W