By using manifold vacuum to operate the piston, the venturi size is always kept correct at any throttle position. During braking, the solenoid valve closes the vacuum and the piston falls freely down. The ECU controls this valve based on the MAP sensor and engine RPM.
Health check
1. Disconnect the vacuum hose from the top of the carburetor, connect a hand pump to the piston and pump out the air. The vacuum should decrease gradually as the piston falls.
2. Start the engine. When the engine is running at idle, 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, during engine operation, air quickly removes the vacuum.
3. Disconnect the #28 hose from the air filter to the piston control solenoid valve and connect a vacuum gauge. At idle, there should be a vacuum. Accelerate the engine to 3000 rpm and quickly release the accelerator pedal. For a short time, the vacuum should drop to almost zero. The solenoid valve is controlled by the ECU during the deceleration of the machine through the yellow (+) and black (-) the wire.