road and workshop
Page 46
Page 47
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by Handyman
Auto-electrics for the mechanic (21)
THE contact breaker gap controls the time the points are closed at any given speed, and there is a sound electrical reason why this timing is vital to top performance. With the contact points closed, the current will flow through the primary coil windings — but because of the self-induction of the winding, the current will not at once reach its maximum value. Therefore time is needed for the current in the primary coil and the surrounding magnetic field to build-up.
In instances where the contact breaker points have been set too wide, the contact points will not remain closed long enough for a satisfactory build-up. Consequently, when the points do open, the primary magnetic field is so weak that the induced voltage inz-the secondary or high tensions coil is so low that it causes the engine to misfire at the higher speeds. Also, an excessively wide contact breaker gap will slightly advance the timing.
Timing is also upset where the points are set too close: the points remain closed too long and this causes a higher current flow. In such circumstances the contact points can be damaged and burned by excessive sparking and the condenser may not be able to prevent this from occurring. The engine will therefore go off tune.
The slightly retarded ignition timing can slow down acceleration fractionally with some added risk of a temperature increase
U sustained full-throttle openings. Accuracy s called for when setting the contact xeaker points.
Inaccurate points settings will often arise where the vehicle is not equipped with a ;tarting handle, and the mechanic relies won placing the vehicle in a high gear with :wake off, and then rocking it backwards or 'orw ards until the points open. This is a real lit-or-miss method, yet it is quite a common ;ight in many garages. There is nothing wrong in moving the engine over in gear to he required point, but the plugs must be temoved to cut out compression resistance and reverse movement. In fact, with the lugs removed it is quite an easy matter to move the engine over by a pull on the fan belt.
Contact points inspection When setting the gap with the contact breaker lever heel on the cam peak, the feeler gauge should be a sliding fit in correctly set contacts. There should be no heavy drag on the gauge blade, otherwise there will be a step down as the blade is withdrawn and the points will be closer than intended. Contact points should be inspected for burned or blackening and should be trimmed with a fine carborundum stone or glasspaper. then wiped clean with a petrol-damped cloth. Any build-up of slag found on the points should be removed and the high spot levelled off; if there is a depression on the points, a new set of contact points should be fitted.