Idle and Acceleration / Deceleration Emission Controls

Fig. 74 Dash pot adjustment:

DASH POT

The carburetor is equipped with a dash pot, Fig. 74 which delays the throttle valve closure to its normal idling position to reduce HC emissions during deceleration.

Fig. 12 Deceleration spark advance system:

DECELERATION SPARK ADVANCE

To reduce HC emissions during deceleration the ignition timing is advanced by a deceleration spark advance system, Fig. 12. Ignition advance is usually controlled by carburetor ported vacuum as well as engine speed. The deceleration spark advance system uses a solenoid valve and engine speed sensor to apply manifold vacuum during deceleration to increase spark advance. During deceleration the vacuum source for the vacuum advance is switched from ported vacuum to manifold vacuum. When the engine speed sensor detects engine speed at or below a specified value the vacuum source is once again switched to ported vacuum for smooth idle performance.

Fig. 46 Throttle opener system (not operated):

THROTTLE OPENER SYSTEM

The throttle opener system consists of a throttle opener assembly, a solenoid valve, an engine speed sensor, and a compressor switch for the air conditioning unit.

When the compressor switch is turned on and when the engine speed sensor detects engine speed at or below the specified minimum idle speed, the solenoid valve is opened, Fig. 46 to transfer intake manifold vacuum to the throttle opener. The throttle valve is then opened slightly by the action of the throttle opener. The increased throttle opening compensates for the increased load on the engine due to engagement of the air conditioning clutch.

Fig. 45 Throttle opener system (operated):

When the compressor switch is turned off, the solenoid valve closes, Fig. 45 and the throttle opener system returns the engine runs to normal idle speed.

The engine speed sensor used in the deceleration spark advance system is used in the throttle opener system, However, when the sensor detects the engine speed at or below the specified value the deceleration spark advance system does not operate.

In the case of the engine speed being above the specified value, the deceleration spark advance system operates normally.

Fig. 23 Tamper-proof idle mixture:

TAMPER RESISTANT IDLE MIXTURE

All carburetors have a tamper resistant idle mixture adjustment, Fig. 23. The CO setting has been made at the factory. Neither removal of the plug or tampering with the mixture screw is required in service except when a major carburetor overhaul, throttle body replacement, or high-idle CO adjustments are required by state or local inspections.

Fig. 24 Tamper-proof choke:

TAMPER RESISTANT AUTOMATIC CHOKE

All carburetors also have tamper-proof choke, Fig. 24. The choke related parts are factory adjusted. Neither removal of the choke cover or tampering with the wax-stroke adjusting screw (WAS) is required in service except when major carburetor overhaul or adjustment of choke calibration related parts is required by state or local inspection.

Fig. 85 Jet valve system operation.:

JET VALVE

In addition to the intake valve and exhaust valve, a jet valve has been provided for drawing jet air (super lean mixture or air) into the combustion chamber. The jet valve assembly consists of the jet valve, jet body and spring, and is screwed into the jet piece which is press-fitted in the cylinder head with its jet opening toward the spark plug.

A jet air passage is provided in the carburetor (or throttle body), intake manifold, and cylinder head. Air flows through the two intake openings provided near the primary throttle valve of the carburetor, goes through the passage in the intake manifold and cylinder head, then flows through the jet valve and the jet opening into the combustion chamber.

The jet valve is actuated by the same cam as the intake valve and by a common rocker arm so that the jet valve and intake valve open and close simultaneously.

On the intake stroke, Fig. 85, the fuel-air mixture flows through the intake valve port into the combustion chamber. At the same time, jet air is forced into the combustion chamber because of the pressure difference produced between the two ends of the jet air passage (between the jet air intake openings in the carburetor throttle bore and the jet opening of the jet piece) as the piston moves down.

When the throttle valve opening is small during idling or light load, a large pressure difference is produced as the piston goes down, causing jet air to flow into the combustion chamber rapidly. The jet air flowing out of the jet opening scavenges the residual gases around the spark plug and creates a good ignition environment. It also produces a strong swirl in the combustion chamber which continues throughout the compression stroke and improves flame propagation after ignition, assuring high combustion efficiency.

When the throttle valve opening is increased, more fuel-air mixture is drawn in from the intake valve port so that the pressure difference is reduced and less jet air forced in.

The jet air swirl dwindles with increase of the throttle valve opening, but the intensified inflow of normal intake fuel-air mixture can satisfactorily promote combustion.