Emergency start?

Yes it will. When the switch is set to emergency start the high and low power coils of the alternator are summed together to max the output. In the old 6V system, however without the battery in circuit, there will be a chance of frying the coil should you rev the engine as the coil is not loading the voltage rise like a battery would. With the 12v system perhaps modified from the 6V one by including the Zener diode there will be a maximum rise to about 14.5V which is OK so you won't fry the 12V coil but once again too much revving or for too long could burn the Zener diode out instead as it will have to sink too much current to earth, well above its design rating.

Bottom line, use it to start the engine but keep revs low only for a short period and,if it will run this way, switch over to normal ignition where you can probably get away with running it for longer without any damage to the coil or Zener diode, if fitted.

Les