I can't understand this effect :
"The suction piston will drop before the induction stroke starts. When induction begins, both the inertia of this piston and the damper will slow the rate it rises. Under these conditions, the carburettor will deliver a richer mixture"
In my understanding, the piston is actually lower than it sould be, so the annulus around the needle is smaller and the mixture should be weaker.
Can you explain this please ?
I am afraid this is another case of reality not matching expectations. There is a second effect with a lower piston height.
The pressure in the choke is reduced which means more petrol enters the airstream for a given annulus. In practice the lower air pressure wins over the reduction in size of the annulus so the mixture gets richer.
This is better explained on page 99 /100 of the book where it describes the effect of different springs.