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road and workshop
Page 80
Page 81
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Auto-electrics for the mechanic (22)
AT THIS STAGE it is worth while spending some time looking at the mechanical side of ignition. Timing is very important to ensure full power, easy starting, and, of course, economy, so the mechanic who can set up the ignition side spot-on every time is an asset to any garage.
When timing an engine by the static timing method, the first task is to see that the contact breaker points are correctly set to the indicated gap. It is usual to select No 1 cylinder for timing purposes, and, with both valves closed in the case of the four-stroke engine, turn the engine towards top dead centre in normal direction of rotation. Timing marks provided by the maker will be found usually on the flywheel rim and housing, but could be also on the crankshaft fan pulley and timing cover. Make sure the correct engine cc is noted, as the degrees of timing advance can be varied between engine sizes.
The engine should be turned carefully until the appropriate timing marks coincide. Then if the distributor has been removed, or the timing disturbed by releasing the locking clamp, the rotor arm should just be opening — as this is not always easy it pays to use a test lamp, which you connect between the primary terminal in the distributor body and earth.
You should then remove the pipe connection to the vacuum advance, and slacken the clamp bolt on the distributor base plate to free the distributor. Switch on the ignition and carefully rotate the distributor body round the cam lobe in the direction of rotor rotation in order to close the points. Next, gently reverse the rotation of the distributor until the test lamp just comes on — this is the exact moment of points opening. Lock the clamp bolt at this point.
As there is usually some backlash in the gears driving the distributor it pays to hold the rotor arm lightly against rotation to take up the small amount of slack in the gears.
The vacuum advance unit plays a big part in engine performance and does, in fact, supplement the mechanical advance mechanism. It is piped to the carburetter and is effected by the suction side of the engine. It comes into play when the engine is at light throttle openings or light load; it will advance the timing as the throttle is eased or the engine overcomes a load, and this gives a little longer burning time for the fuel. The vacuum unit consists of a spring-loaded flexible diaphragm which is linked to the plate carrying the contact assembly. As the diaphragm responds to changes of vacuum in the inlet manifold, so does the linkage move the contact breaker plate in the direction of advance or retard.
When the throttle is eased and the vacuum effect rises, so will the contact plate be moved towards direction of cam rotation and will advance the ignition timing. As the throttle is opened and the vacuum value drops away, so will the spring tension return the timing to normal position.
The pipe from the centre of the diaphragm unit is connected at its other end into the carburetter inlet just on the atmospheric side of the butterfly throttle valve.