This system is designed to reduce the level of hydrocarbon emission during rapid throttle closure at high speed. It is used on some 49 state and all California models, with a V8 engine.
The system consists of a deceleration valve located at the right front of the intake manifold, and a throttle modulating diaphragm located at the carburetor base. The valve and the diaphragm are connected by a vacuum hose and the valve is connected to direct manifold vacuum. During deceleration, manifold vacuum acts to delay, slightly, the closing of the throttle plate.To adjust
- Run the engine to normal operating temperature and set the idle speed to specification. Shut off the engine.
- Position the throttle lever against the curb idle adjusting screw.
- Measure the clearance between the throttle modulating diaphragm plunger and the throttle lever. A clearance of 1 / 16 in. (1.6mm) should exist.
- Adjust the clearance, if necessary, by loosening the jam nut and turning the diaphragm assembly.