Starter enrichment valve
From OpenContent Curriculum
The Starting Enriching System
The "choke" isn't. An actual choke in a carburetor restricts air flow and causes the "suction" (lower relative air pressure) to pull more fuel from the bowl creating a very rich condition. The starter enriching feature on these Mikuni carburetors uses a plunger to open or close an extra jet. Normally it is closed, but can be opened when the engine is cold in order to provide the necessary extra fuel a cold engine needs. The advantage of using a jet system is that a jet works, in conjunction with air passing through, to vaporize the fuel, which creates better conditions for the fuel to ignite. Still, we often use the term "choke".
The starter enriching system uses the plunger pictured above to control the air and fuel ports in this circuit to allow or prevent a fuel-rich starting condition. The metal cylinder covers the air port, while a soft synthetic seal on the bottom of the plunger covers the fuel inlet port.
The air intake port (left/upper pic) is found in the intake side of the carburetor. The blue line (right/lower pic) shows the air intake passage from the outside. The orange line shows the approximate path the fuel follows within the carb. The circles indicate the area under the plunger where the air and fuel meet and mix as they are carried to a port which feeds directly into the intake ports on the engine.
Another view seen 90º to the previous pic. It is a bit easier to visualize the path of the fuel (orange lines) from the float bowl.
The plunger seat and area (left/upper) where air and fuel meet and mix before being carried through the starter enriching passage which exits into the carb throat attached to the intake port on the engine. The large round hole on the left-hand side of the throat (right/lower pic) is the exit port for the starter enriching circuit. It provides the very fuel-rich mixture which enables the engine to start, even in cold weather.