Fitting Thorspark electronic ignition then removing it and fitting Electrex ignition and charging.

The Interceptor ignition was originally by a Lucas K2F magneto. I have never had much luck with magnetos so I did a little research and found the Thorspark electronic ignition system from Sussex Motorcycles. The system cost £150. The ignition trigger fits in the magneto end cap in place of the points. You have to enlarge the drain hole in the end cap a little for the three wires to come out of. The existing tool box on the Interceptor is huge and can be used to hold the ignition black box and the dual out put coil. A modern 12v AGM type battery is mounted in the existing battery box. I also added an ignition switch and a fuse. There is no fuse at all in the original wiring loom!

The directions with the Thorspark are good but it is not quite clear how to set up the little rotor inside the end cap. The instructions say to set it so that at the point of full advance (32 degree btdc in this case) it is approaching the encapsulated sensor that you can see on the left.

After some experimenting and some strong kick backs I positioned the rotor as shown in the photo that goes with this post. You have to undo the screw in the middle each time and move the rotor a little to change the point at which the magnet passes the rotor. Each time you do this make sure the full advance mark is lined up on the alternator.

The bike starts easily now and seems to rev cleanly but I need a second pair of hands in the workshop to connect up a strobe and check it is advancing OK.

The alternator has been converted to provide 12v by connecting the green\black and green\yellow wires together into one bullet connector and leaving the green\white wire on its own. These make two connections which are connected to the two yellow wires on the Podtronics regulator\rectifier mounted beneath the battery box where the old zener diode was.

Running the engine and revving it shows over 14v at the battery so the 50 years old alternator is working OK. For now…

Update: 

The only drawback of using this system is it depends on the accurate rotation of the old magneto. On the Interceptor the magneto is on the end of a chain running off the inlet camshaft. As this wears it introduces slack in the system. In my mind this is not ideal.

After setting up the Thorspark system I found the bike would still start OK but sometimes run a bit “off” Then a week later it would run OK. I think the worn out mag chain and mag internals allowed the timing to change a little.

The best place for the ignition system is on the end of the crank I think. Enfield did eventually change the Interceptor ignition to use points mounted on the end of one of the camshafts. I therefore bought the Electrex system which provides the sparks and can also charge the battery.

The bike starts well and more importantly it runs consistently well. Nothing else has been changed.

I think the Thorspark would be a good option for a gear driven magneto.