Motorcycle plunger carburetor does not have a cut-off valve, so why does the quick return throttle not turn off, while scooter vacuum membrane carburetor must have a cut-off valve?

The air cut-off valve you mentioned is called rotary valve on motorcycles and butterfly valve in industry.

In the vacuum carburetor, the rotary valve is the main throttle valve to control air intake, and the plunger is the driven throttle valve and the oil needle opening control valve.

In the ordinary plunger carburetor, the plunger is a throttle valve, which may also be called a plunger valve, but its structural principle is called a gate valve (that is, a valve like a gate).

When the ordinary plunger carburetor returns to the throttle quickly, it only makes the plunger drop quickly (note that it will not fall to the bottom at any time, even if it is completely released). The engine will continue to rotate and decelerate due to inertia. At this time, it still sucks air and engine oil from the carburetor, but this air volume is less because of the small opening of the valve. When the engine speed drops to a certain extent, a small amount of mixed steam inhaled can make it continue to work, and it will stop working normally at this speed.

One of the adjustment requirements of carburetor idle speed is no stopping, no flameout, no tempering, no flameout and no idle drift.