The Boost Control Deceleration Device (BCDD) is used on gasoline-powered (only) L-series engines to reduce hydrocarbon emissions during coasting conditions.
High manifold vacuum during coasting prevents the complete combustion of the air/fuel mixture because of the reduced amount of air. This condition will result in a large amount of HC emission. Enriching the air/fuel mixture for a short time (during the high vacuum condition) will reduce the emission of the HC.
However, enriching the air/fuel mixture with only the mixture adjusting screw will cause poor engine idle or invite an increase in the carbon monoxide (CO) content of the exhaust gases. The BCDD consists of an independent system that kicks in when the engine is coasting and enriches the air/fuel mixture, which reduces the hydrocarbon content of the exhaust gases. This is accomplished without adversely affecting engine idle and the carbon monoxide content of the exhaust gases.
Normally, the BCDD does not need adjustment. However, if the need should arise because of suspected malfunction of the system, proceed as follows:
- Connect the tachometer to the engine.
- Connect a quick response vacuum gauge to the intake manifold.
- Disconnect the solenoid valve electrical leads.
- Start and warm up the engine until it reaches normal operating temperature.
- Adjust the idle speed to the proper specification.
- Raise the engine speed to 3,000-3,500 rpm under no-load (transmission in Neutral or Park), then allow the throttle to close quickly. Take notice as to whether or not the engine rpm returns to idle speed and if it does, how long the fall in rpm is interrupted before it reaches idle speed.
At the moment the throttle is snapped closed at high engine rpm and the vacuum in the intake manifold reaches between 23 in. Hg and 27.7 in. Hg and then gradually falls to about 15.6 in. Hg at idle speed. The process of the fall of the intake manifold vacuum and the engine rpm will take one of the following three forms:
- When the operating pressure of the BCDD is too high, the system remains inoperative, and the vacuum in the intake manifold decreases without interruption just like that of an engine without a BCDD.
- When the operating pressure is lower than that of the case given above, but still higher than the proper set pressure, the fall of vacuum in the intake manifold is interrupted and kept constant at a certain level (operating pressure) for about one second and then gradually falls down to the normal vacuum at idle speed.
- When the set of operating pressure of the BCDD is lower than the intake manifold vacuum when the throttle is suddenly released, the engine speed will not lower to idle speed.
To adjust the set operating pressure of the BCDD, remove the adjusting screw cover from the BCDD mechanism mounted on the side of the carburetor. On 810 models, the BCDD system is installed under the throttle chamber.
The adjusting screw is a left-hand threaded screw. Late models may have an adjusting nut instead of a screw. Turning the screw 1 / 8 of a turn in either direction will change the operation pressure about 0.8 in. Hg. Turning the screw counterclockwise will increase the amount of vacuum needed to operate the mechanism. Turning the screw clockwise will decrease the amount of vacuum needed to operate the mechanism.
The operating pressure for the BCDD on most models should be between 19.9 to 22.05 in. Hg. The decrease in intake manifold vacuum should be interrupted at these levels for about one second when the BCDD is operating correctly.
Don't forget to install the adjusting screw cover after the system is adjusted.