I’ve run an Interspan ignition for about 30 years on my NRE Weslake twin and fortunately I haven’t had to think too hard about setting up the trigger timing since, like Chis, I just line up the scribed line on the pickup coil assembly with the scribed line labelled TDC on the rotor. When I did try and figure out why the magnets were where they were I got confused pretty quickly.
I have Interspan rotors for the Weslake engine and one for a Commando engine and these engines have the cam turning in opposite directions ( when viewed from the timing side, cw for the Weslake, anti-cw for the Norton)
On the Weslake rotor the centerline of the magnet hole is about .15“ below the scribed TDC line (with the scribed line at the 9 o’clock position) which implies the magnet centre is actually passing the centre of the pick-up coil pole piece after TDC
Similarly on the Norton rotor, the centerline of the magnet hole is about .18“ above the scribed TDC line ( with the scribed line at the 9 o’clock position) which also implies the magnet centre is actually passing the centre of the pick-up coil pole piece after TDC. Given the different cam direction this is consistent with the Weslake set up, although the Norton does have a bigger magnet hole as do the later Weslake rotors.
It seems from the above that there is enough magnetic field strength as the magnet approaches the coil pole that it generates enough voltage in the coil to trigger the ignition at the right number of degrees before TDC.
It also occurs to me that with a single you might need to pay attention to whether you’re timing it on the compression stroke if you only have 1 magnet on the rotor (assuming on the RE, the rotor is driven from the cam) My rotors are cam driven but have 2 magnets 180 degrees apart so it fires like a conventional 360 twin with wasted spark. Hence it doesn’t matter if you time it on compression or exhaust stroke. I might matter on your setup. Given that the engine was working and that you had made a note of the rotor position before pulling it apart this might account for your problems.
I hope the above might help and you’ll get it going again.
Duncan