Messo in D o R calo di giri

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Da shop manual ('68)

 

1 DESCRIPTION AND OPERATION


IGNITION SYSTEM


The 1968 engine ignition systems are basically the same as in past model years, but some new features have been incorporated. Improvements to the distributor timing advance and retard mechanisms, in combination with the leaner carburetor air-fuel ratios, provide for the required reduction in exhaust gas contaminants on Imco engines.


Thermactor engines employ additional means of exhaust emissions control. New features in the ignition system are:


Dual Diaphragm Vacuum Advance


Distributor


Distributor Vacuum Control Valve


Distributor Vacuum Advance Control Valve


Vacuum Sources for Vacuum Advance Units Distributor


All distributors are equipped with both vacuum and centrifugal advance units. The vacuum advance governs the ignition timing (spark advance) during low engine speeds (rpm) or low engine loadings. The centrifugal advance, in combination with the vacuum advance, control the ignition timing at higher engine speeds or heavy engine loadings to provide the correct ignition timing for maximum engine performance. A dual-diaphragm vacuum advance is used on some engines to provide additional ignition timing retard during engine operation.


 

DUAL ADVANCE DISTRIBUTOR—SINGLEDIAPHRAGM VACUUM ADVANCE


The distributor has two independently operated spark advance systems. A centrifugal advance mechanism (Fig. 1), located below the stationary sub-plate assembly, has centrifugal weights that move inward or outward with changes in engine speed. As engine speed increases the centrifugal weights cause the cam to advance or move ahead with respect


to the distributor drive shaft. The rate of advance is controlled by calibrated weight springs.


The vacuum advance has a spring loaded diaphragm connected to the breaker plate assembly. The diaphragm is actuated against the spring pressure by vacuum pressures. When the vacuum increases, the diaphragm causes the movable breaker plate to pivot on the stationary sub-plate. The breaker point rubbing block, which is positioned on the opposite side of the cam from the pivot pin, then moves opposite to distributor rotation and advances the spark timing. As the movable breaker plate is rotated from retard position to full-advance position, the breaker point dwell decreases


slightly. This is caused by the breaker point rubbing block and the cam rotating on different axes.


 

DISTRIBUTOR DUALDIAPHRAGM VACUUM ADVANCE


On dual-diaphragm vacuum advance distributors, the centrifugal advance unit is the same as on single-diaphragm


vacuum advance distributors. The dual-diaphragm unit consists of two independently operating diaphragms (Fig. 2). The outer (primary) diaphragm utilizes carburetor vacuum to advance ignition timing. The inner (secondary) diaphragm is actuated by intake, manifold vacuum to provide additional ignition timing retard during periods of closed throttle deceleration and idle, thereby assisting in the reduction of exhaust system hydrocarbons.


The outer diaphragm is coupled to the movable breaker plate much the same way as in single-diaphragm distributors.


An increase in vacuum pressure moves the diaphragm against the advance diaphragm spring tension, causing the movable breaker plate to pivot opposite to distributor rotation. Thus, an ignition timing is advanced, and this is calibrated to occur during normal road-load operation, but not during deceleration or idle. When intake manifold vacuum is applied to the inner diaphragm (retard), it moves inward toward the distributor. This allows the advance diaphragm spring to move the advance diaphragm causing the movable breaker plate to pivot in the same direction as distributor rotation. This retard of the ignition timing occurs during engine idle or deceleration


 

DISTRIBUTOR VACUUM CONTROL VALVE


The distributor vacuum control valve (temperature sensing valve) is incorporated in the distributor vacuum advance supply line to provide advanced ignition timing under certain engine operating conditions. The distributor vacuum control valve is installed in the coolant outlet elbow to sense engine coolant temperatures. Normally, the valve connects two ports, normal source vacuum at the carburetor and the distributor port. During periods of prolonged idle, should the engine temperature rise above normal, the valve closes the normal source vacuum port and connects the distributor port to the alternate source vacuum port. On some engines the latter port is capped with a filter, thus sensing ambient pressures. The advanced ignition timing causes an immediate increase in engine speed which will continue until the engine temperature returns to normal. Distributor vacuum system schematics for the various engine systems are shown in Figs. 32 thru 43 (Part 1 of this manual).


 

DISTRIBUTOR VACUUM ADVANCE CONTROL VALVE


On the 240 six-cylinder engine, a distributor vacuum advance control valve (deceleration valve) is incorporated in the distributor vacuum system to provide additional control of the ignition timing. This device is used in conjunction with the dual-diaphragm vacuum advance unit. Normally, the outer (advance) diaphragm is connected to a vacuum port on the carburetor. During deceleration periods when intake manifold rises above a specific valve, the deceleration valve


closes off the carburetor vacuum and provides direct intake manifold vacuum to the distributor outer diaphragm.


This permits maximum ignition timing advance to prevent afterburning or popping in the engine exhaust system. When the vehicle slows down and the engine is operating at idle, the deceleration valve shuts off the intake manifold vacuum


and opens the carburetor vacuum to the distributor.