I loved my D14/4 - my first bike. That looks very nice. Good description from Fawlty.
The iginition timing was fixed and was always a compromise between being too sluggish and having better acceleration but horrible engine rattle / pinking. Easy to experiment and adjust at the side of the road though - behind that circular cover near the gear lever. (British bikes typically had their brake pedal on the left and gear lever on the right).
There was no ignition key, ignition was turned on by one of the switches on the headlight.