By using manifold vacuum to operate the piston, the venturi tube size is always maintained at the correct throttle position. During braking, a solenoid valve closes the vacuum and the piston falls freely. The ECU controls this valve based on MAP sensor readings and engine speed.
Checking functionality
1. Disconnect the vacuum hose from the top of the carburetor, attach a hand pump to the piston and pump out the air. The vacuum should gradually decrease as the piston drops.
2. Start the engine. When the engine is warm and idling, the piston should not hold a vacuum. Inside the tube on top of the piston there is an air connection to the air intake manifold, through which, when the engine is running, the air quickly removes the vacuum.
3. Disconnect hose #28 from the air cleaner, leading to the piston control solenoid valve, and connect a vacuum gauge. There should be a vacuum there at idle. Accelerate the engine to 3000 rpm and quickly release the accelerator pedal. The vacuum should drop to almost zero for a short time. The solenoid valve is controlled by the ECU during braking of the car via a yellow (+) and black (-) wire.
(The article was copied from the website: «hondabook.ru»)