Hi Peter,
The strobe usually works, sometimes it's a bit intermittant. I guess it's all to do with how good the impulse pickup is on the spark plug wire.
My strobe light is a simple cheapish power strobe bought from an aftermarket auto parts outlet. This particular strobe does not have the extra wire you speak of, but I had another that did have this wire. There was an analogue dial on the strobe gun, the extra wire went to the distributer and you could check dwell angle. Don't know how or if this would work with a magneto.
I have a 1994 Harley davidson Electraglide, evolution motor. These motors had two methods of sparking the plugs, both plugs together with a waste spark on the exhaust and both individually (seperately. Mine fires both at the same time. They are a terrible motor to set ignition timing on with a strobe light, but mine will not work on the rear cylinder, only the front. I do have a magneto on another Norton that the strobe won't work on as well. Maybe something to do with type of spark plug wire.
I bought an electronic tacho (engine revs) that was supposed to work on these engines. The type that is used on go carts and other motors with self energising ignition. Unit has its own small battery, like a bicycle speedo and a wirethat wraps around the spark plug lead. I could never get it to work properly. Readings would be all over the place....!!!
Since a strobe is only going to give you standard timing and as fuel is now different now I can't see the point. My M21 was rebuilt as a magazine project bike with the the timing set exactly. It was wrong. I had to advance it a bit to get it running just so.
Oh and whilst the military specified 50 for the engine Pitman quotes 40 for winter and 60 for summer. The army just standardised not optimised. I don't know if Morris do a straight 60
