Piston Engine Deceleration Control System

ADJUSTMENTS

Vacuum Throttle Opener

1978 GLC

1. Connect a tachometer to the engine. Run the engine until hot, then shut it OFF and remove the air cleaner assembly.

   Fig. 1: Adjusting the vacuum throttle opener system — 1978 GLC

   Fig. 2: Disconnect/plug the anti-afterburn valve vacuum line, then tee a gauge between the intake manifold and vacuum control valve — 1978 GLC

2. Disconnect the vacuum sensing tube F from the servo diaphragm, then connect a vacuum hose between the vacuum tap on the intake manifold and the diaphragm.
3. Disconnect the vacuum line that runs from the carburetor to the distributor at the distributor, and plug the open end.
4. Start the engine and read the tachometer. Engine speed should be 1,300–1,500 rpm. If the speed is not to specification, turn the throttle opener adjusting screw to bring engine speed within the range.
5. Reconnect the distributor and servo diaphragm vacuum lines, then disconnect and plug the vacuum line going to the anti-afterburn valve. Disconnect the vacuum line going from the intake manifold to the vacuum control valve, and tee in a vacuum gauge as shown.
6. Start the engine and accelerate to 3,000 rpm. Watch the vacuum gauge and release the throttle. After a rapid rise in vacuum, the gauge reading should stabilize at 22.0–22.8 in. Hg (559–579mm Hg) for a few seconds while the system gradually closes the throttle, then drop off.
7. If vacuum does not stabilize in the right range, loosen the locknut and turn the adjusting screw in the end of the vacuum control valve until vacuum is in the specified range. Turn the screw clockwise to increase the vacuum reading, and counterclockwise to decrease it.
8. Tighten the locknut, then restore all vacuum connections. Install the air cleaner assembly and remove the tachometer.

Servo Diaphragm

1. Connect a tachometer to the engine.
2. Run the engine at idle to normal operating temperature.
3. Stop the engine and remove the air cleaner assembly.
4. Disconnect the vacuum sensing tube at the servo diaphragm.
5. Connect the inlet manifold and the servo diaphragm with a suitable tube, so that the inlet manifold vacuum can be led directly to the servo diaphragm.
6. Disconnect the vacuum sensing tube (which runs from the carburetor to the distributor) at the distributor, then plug the tube.
7. Start the engine and check to see that the engine speed increases to 1,100–1,300 rpm (GLC) or 1,000–1,200 rpm (626).
8. Turn the throttle positioner adjuster screw in or out, as required, to adjust to specifications.