The SP P1's, at least around 1928-29, also had the separate starting carb, but it was operated, as I recall, by a lever on the 'dashboard'. The primary carb had a mixture control on the steering wheel hub on top; at full rich it also injected oil to deal with oil runoff when the car was shut down. As you moved the mixture lever to the right, you could feel a bit of extra resistance. This all led to a fairly crazy starting sequence, for anyone who doesn't know it (cold engine). Mixture to full rich. Hand throttle open, spark retarded. Key turned, spark lever to both, depress starter button with your foot. Engine catches, you lift your foot, start to advance, after a few seconds turn off the starting carb, start to ease off the mixture. Continue to engine smooth, mixture to reflect what altitude you were at. That's it, more or less, multiple pages in the owners manual to get through the sequence. And of course, don't forget a pull on the Bijur, preferably on the street, not your garage floor or driveway.
Engine warm, you MIGHT get away with turn key, ignition to both, throttle open, mixture a bit rich, flick spark advance lever up and down, mixture in cylinders ignites, engine starts.
Taught myself with the help of the manual, and things I heard at car shows! 🙂