When the engine is cold fuel tends to condense on the walls of the intake manifold and cylinder and this make the engine hard to start.
The solution is to make the mixture richer, either by temporarily adding more fuel or by restricting the amount of air. The mechanism used on this carburettor is a simple butterfly choke that restricts the amount of air when it is used. Here are the bits involved:
The choke plate pivots on a pin inserted through the opening on the filter side . The pin is rotated by a lever on the side of the carburettor – on the z2 model the lever is attached by a cable to a knob situated on the plastic cowl below the handlebars. When the knob is pulled out it pulls on the lever and shuts the choke plate:
As we saw in the previous post, when the bike idles without the choke the engine is pulling in mixture from the pilot hole and air through the venturi at the same time. When the choke plate is closed it reduces the amount of air that can be delivered via the venturi and increases the suction on the pilot hole resulting in a richer mixture.
There are two springs on the actuating lever: the large one returns plate to the open position when the choke knob is pushed back in. There is a much smaller spring that holds the plate vertically against the stop in the carburettor body. As far as I can work out the purpose of the small spring is to provide just enough resistance to keep the plate vertical but not so much that it can’t be sucked open when more air is needed (e.g if the rider blips the throttle after the engine starts).
Not much can go wrong with this type of choke, unless it get stuck for some reason (with obvious consequences).
You may be relieved to hear that is the last post on the Keihin PB25 carb!