hi may be a silly question. with ignition switch on
bike the central position is off. there are two other
position ign on and emg on.
From the '65 B31/B33 (which appear to have the same switch) Instruction manual
on Kim The CD
Man's BSA CD
In the centre of the lighting switch on the headlamp cowl. It has three positions: Off, Ignition and Emergency. Insert the ignition key and turn to the right before attempting to start the engine. If, for any reason, the battery is exhausted turn the switch to the left then start the engine in the normal way. As soon as the engine is running at a moderate speed rotate the switch rapidly to the normal ignition position when it should continue to run normally.".
think the emergency position bypasses battery and takes feed from stator direct.
Not really. In 6V systems, the battery (crudely) performs the task of volt/current regulation. Any Lucas
single-phase alternator has six stator coils, connected together as a pair and four. When the bike's running without lights, the pair of stator coils generally provide enough power to run the ignition and keep the battery charged for times of low-rpm running, when the two stator coils don't provide enough power on their own to run even the ignition.
Using the lights obviously requires more power so, when the lighting switch turns them on, it also connects in the other four stator coils. The "Emergency" position also does this but, if you aren't using the lights, once the additional power's charged the battery, it'll simply go on to overcharge it, eventually boiling it dry.
when fitted is the key postion switch down on. and key turned up emergency.
A picture in the aforementioned manual
shows the key slot vertical.