K2FC Earth

Hello again Robert - the later steel points plate don't have the carbon earthing brush like the brass ones from earlier bikes. So I presume that they aren't really necessary.  To convert it you'll need to remove the cable and fit a modified bolt into the cable orifice to keep the cam ring in a fixed position.  There again you could remove the cam ring and fit one from an auto mag. I am sure you'll have one somewhere in you stash.   Good luck Howard

If the manual setup is fixed on the sweet spot at full advance, there's no reason not to simply add an auto advance unit.  If you can find one.  I believe Triumph ones are not too difficult to find but operate in the opposite direction?  Might be wrong.
The earth brush in the centre of the end cap is the only way to stop the engine when you have a Twin without the decompression lever usually fitted to a Single.  Otherwise you'd have to stall it against the clutch or turn off the fuel tap and wait.
in theory, a bike should start more easily with auto advance because the mag fires at the same spot regardless of the timing.  It would be interesting to fit an auto advance to my Inter, but there isn't enough space on the Singles.  Same applies for your Rudge?

"to have both systems operating on one mag surely the manual would be set at retard"

Definitely not!  The movable cam ring would be in it's advanced position in the housing, so that the spark is strong.

The magneto sparks at the point where the contact breaker separates.  This strongest spark is when the magnetic field collapses before it builds up again to the opposite polarity after the armature turns another 180 degrees.  This point is usually arranged to be when the contact breaker is rotated to the full advance position.  If it is fixed at full retard, it will stay there forever and be at a weaker magnetic field point and give a weaker spark forever.  This is the problem with manual magneto.  The automatic advance doesn't alter the relationship between the contact breaker and the magnetic field, so it always gives the strongest possible spark regardless of timing.

I don't know why they don't arrange it to give the best spark when at least part retarded, to make it easier to start.  The reason must be to get the best performance out of the magneto when on the road.

I understand the ignition issue with Vincent twins is that the magneto sparks better on one cylinder than on the other.  This is because the best spark positions are 180 degrees apart, but a V twin sparks at some odd angle.  So only one cylinder sees the 'best' spark, and they tend to fire up on that 'best' cylinder.  Perhaps an owner might confirm?