MAGNETOTECH
If a magneto-equipped machine is hard to start when it’s hot, you probably have electrical issues. Neil Cairns experiments with a quick-fit condenser / capacitor replacement
If a magneto-equipped machine is hard to start when it’s hot, you probably have electrical issues. Neil Cairns experiments with a quick-fit condenser / capacitor replacement
All three of my AMC machines have magnetos. Both of my AJS 18S singles have Lucas N1 mags and the Matchless G12 650 twin has a Lucas K2F. All are of the ‘rotating armature’ type with the ignition points on one end rotating with the armature on a face-cam. All three bikes are of the 1950s when the dynamo originally supplied lighting. I’ve fitted 12V Alton ‘permanent magnet’ alternators that look just like a dynamo.
Last year the G12 began to misfire badly on my return trip from the Shuttleworth Collection. The condenser was failing so
we stuttered and banged our way home at 20mph. The condenser lives inside the windings of the armature (the armature is the bit with the copper windings on), so it is not a simple job to replace, and the armature must be rewound. I sent it off to be fixed and it was returned within the week in good order.
A condenser stops the ignition points from sparking and quickly eroding away, as well as absorbing then releasing some power to assist a better, cleaner spark. If it breaks down internally it sends out spurious electrical signals or simply goes to earth, causing misfiring or no spark at all. On ‘rotating magnet’ magnetos it is possible to slave-fit a condenser from a car’s coil ignition system to get you home, and then simply remove the old one and fit the new one. But not in my case… On my 1952 MG YB saloon, using cheap ‘Made In China’ condensers at £1.50 each proved a false economy: they failed constantly. I eventually purchased a ‘Made In England’ one for £8.
This summer I attended a military ceremony to raise the flag for Armed Forces Week. I’m a rider in the Bedfordshire Clangers, the local Royal British Legion Riders group (see rblr.co.uk). I was a bit early and stopped for a cup of tea at a layby caravan on the A6. When I went to restart my 1953 AJS 18S combination it was dead. Not a single pop or bang. I cleaned the magneto points and the
spark plug. There was a weak orange spark. I let the engine cool and it eventually started up, but I was now late. After the ceremony the engine had cooled down completely and it fired up first kick. This hot starting failure is a sure sign of a failing condenser in the magneto. I rode home making sure not to let the engine stop. That afternoon I ordered a little ‘Brightspark Easy Cap’ condenser from the AJS & MOC spares shop online. It arrived promptly; excellent service, and cost just £13 plus delivery.
I wheeled the combo out of the garage, placed my wife’s gardening kneeler pad on the concrete and removed the rotor and points from the magneto. I cleaned up the bits. On the garage bench in the vice, I removed the Paxaline insulator from betwe een the points mounting and the rotor, fitting th he little ‘CO3’ electronic replacement in its plac ce. It was just too easy.
Upon refitting, I ensured that the little brass locking plate under the long setscrew was in place, to lock the screw’s head against vibration undoing it. This set-screw is responsible for both locating and securing the points and rotor to the end of the armature as well as making the electrical contact for both. A few years ago I had a lot of misfiring and found this tiny brass plate missing; the set-screw had loosened off over the years causing a poor electrical circuit. I held the rotor and found I could rock it about.
With the points cleaned up (faulty au ty condensers cause points to burn), the face-cam lubricated, and the new minute electronic condenser in place, I checked the points gap then kickstarted the bike with the lead on a spare plug. I had a thick, strong blue spark. It fired up third kick and ticked over like a watch. Total time about 20 minutes, total cost about £18.