5. The Carburettor Throttle

This is the front of the carburettor, and you can see all the way through the airway because the throttle is fully open.
The vertical line is the needle valve, which governs the fuel flow.


You can see that the end of the needle valve has a point but it is also delicately tapered.
This is what increases and decreases the flow of fuel into the air flow.

As the needle rises, the taper increases the amount of fuel that can be sucked up into the mixture, and conversely as the needle is lowered it cuts down on the amount of fuel that can be used.