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This is dash pod,
It holds and release the throttle plate aka blade slowly using vacuum...
Copied from net:
"A carburetor dashpot prevents the engine from decelerating too quickly. It is composed of a cylinder, spring and a shaft that is attached to the throttle. When a driver depresses the gas pedal, the spring applies pressure to the shaft, causing it to close the throttle lever slowly. This movement allows the vehicle to return to idle smoothly and prevents the emission of high levels of carbon monoxide. Without the dashpot, the throttle lever would close abruptly, possibly causing the engine to stall due to an abrupt decrease in speed."
Dashpod usually is attached at the right side of carburator, vacuum hose is attached to it...
It can be adjusted by rotating the screw on top of it...