MOTORDOM

Greymouth Evening Star, 10 December 1937, Page 9

MOTORDOM

NEWS OF THE ROAD

(By

“Gearbox”)

PRE-IGNITION.

A COMMON TROUBLE

A worrying trouble which was prevalent in . the early days of motoring and lately seems to be making" frequent reappearances, is pre-ignition. This, is manifested by the engine continuing to fire, and run for few revolutions after it has been switched off. There are other symptoms which are not so easily diagnosed, although the owner is very well aware that the engine is not running as it should. Pre-ignition has nothing to do with detonation. It is the firing of the combustible mixture at the wrong time, not by the spark jumping the plug gap, but by some foreign particle or portion of the cylinder head mechanism becoming overheated and attaining incandescence, so that the mixture is actually fired; at an early point in the upward travel of the piston before the spark at the plug points occurs. « Besides causing the engine to run for a few revolutions after thei ignition has been switched off, this too early firing causes thudding and roughness in the running of the engine. Whether or not this is caused by pre-ignitioni can often 1 be determined by switching off and allowing the engine to cool for a while. On starting again, and preferably travelling at a reduced .speed so that the combustion chambers do not attain the same operating temperature as before, the trouble will disappear if pre-ignition is the cause. One of the most common causes of pre-ignition is an excess of carbon deposits in the cylinders. It is possible for a particle of carbon to stand out from the resit so that 1 most of it is exposed to the heat of combustion. This makes it incandescently hot.

Very frequently the trouble is induced by using a hot sparkling plug in a hot engine. Thei points of the plug, having insufficient resistance to the heat in which they are attempting to work, glow and fire the mixture without reference to the spark, which later jumps across them. Another incidental result of using wrong plugs of this sort is that the points also burn away with extreme rapidity. For these reasons it is essential that some care be taken in choosing plug replacements, the manufacturer’s advice, in his plug chart, being the best to follow. It is possible, however, for plugs of the correct type to become overheated if leaks develop through the threads in the cylinder block into which the plug is screwed. A new copper asbestos washer should effect a cure in this case. When the plug is of the type that can be taken apart a leak between the porcelain insulator and the body, of the plug will have a similar effect. Here, again, a new copper and asbestos washer beneath the collar nut will prevent. the trouble recurring, and is far, better than trying to tighten a worn washer. / A condition which may be very difficult to diagnose is that caused by a faulty cylinder head gasket. In tightening the head it is ! possible to split the rim of the gasket so that a tiny portion of the metal projects into the combustion space, where, under the influence of the prevailing' temperature, it may become incandescent and usurp the duties of the sparking plug. In all case's of pre-ignition, whatever the cause, if ignition occurs before the inlet valve has; closed there will be popping back in the carburettor. If it occurs after the valve has closed the result will be unusually harsh running.

About the last remaining cause of pre-ignition is general overheating in an old 1 engine, in which the water passages have become 'thickly encrusted with lime-deposits, which seriously interfere with the flow of the cooling water. These deposits are particularly apt to occur in the smaller passages, and as these are generally those around the valve heads, the effect of cutting down the water supply' can be very important. At no time' do the exhaust valves operate at as' cool a temperature as is desirable, but if the flow of the cooling water is restricted the exhaust valves 'will tend to become red l hot and will cause preignition while they last. Their life, however, will not be very long '>*jn these conditions, as the heads will burn and the valves) will have to be replaced. This is one of the reasons why radiators and cooling systems' should be flushed at least once a year. SPARE ELECTRICAL PARTS. It is a rather, remarkable thing that in most standard ears no provision is made in the equipment for spare parts for the electrical equipment. The motorist of experience will not think of going far afield without at least a few spare parts with which to cope l with minor troubles.

The! parts which should l be carried’ consist, of spare headlight and tail light bulbs, a box of five spare fuses, and a roll of insulating tape. 11l purchasing these spares care must be exercised! to see that those suitable to the car are obtained. Fusiesi of less than the required amperage .will not be of much help. Tn addition to these spare parts, a sharp knife and! a very small screwdriver, capable of fitting the tiny screws used in electrical connections, should be included in the kit, while at least two spare sparking plugs, again of the right type, will be useful additions. Most of these electricla spares are of rather a delicate nature, and must be stowed in the car very carefully to prevent their being damaged. They can. be kept in a small tin cigarette box lined with felt, but it is far better' to arrange special sockets for the sparking plugs and lamp bulbs, sb that-they are held in a similar manner to thosewhich arc in service. For example, a small hardwood block may be drilled and' tapped with the same threads as' the plugs l and screwed to some convenient, place, such as the engine side of the dashboard. Here the plugs will stay till needed with no possibility of damage. A good idea was passed on the other day by a motorist who makes one of his spare bulbs' serve a. double' purpose. He mounted it in the position' described, but also wired it to the ammeter with a. small switch interposed, and thus was able to light up the engine compartment at night for small adjustments needed. The fuses' and insulating tape ate not particularly fragile, and can be carried

safely in one of the door pockets. It is commonly believed that a blown fuse indicates beyond any doubt a fault in the wiring somewhere, and many motorists 1 start on a hunt for trouble before replacing with a new one. But it seems certain! that fuses often go simply through old age or a temporary heavy consumption! of current, and it has been found that it is worth while to replace a blown fuse straight away with a new one and await developments. If the new fuse takes the load satisfactorily there cannot be much wrong. It is only when a replacement blows straight away that a search for frayed wires and short circuit should be made. In the event of a fuse blowing, with no spares in the car, a temporary repair can be made by wrapping tinfoil from a cigarette packet round the old one and replacing it in its clips. Another idea is to connect the two metai ends) with ordinary fuse wire. These makeshifts, however, should never.be ileft in place permanently, but at the first opportunity a box of fuses should be purchased and one placed in the clips. The old expedient of hooking a liair'pin across the fuse clips is dangerions, because if the wiring is overloaded this will be too strong to 'blow, and the next weakest; link in ! the electrical chain, probably the 1 lamp bulbs, will give way under the I strain of the excessive current.

ELECTRICAL EQUIPMENT.

PREVENTIVE SERVICING

In analyses prepared by automobile associations throughout tile world in reference to mechanical trouble which has caused! motorists to need assistance, ignition is a prolific offender. Although that is a fact large numbers of motorists do not appreciate the value of preventive and curative electrical servicing. In a few weeks many thousands of motorists will be setting out on long tours and submitting their cars , and the electrical equipment to harder work than usual. Give the electrical equipment a thought in advance. Don’t wait till it calls “enough”—miles from home. The electrical installation on the modern car is a remarkably reliable assembly o (appliances. It is a miniature central electric power station, but it works under extremely adverse conditions.

It is required to operate for indefinite periods without skilled 1 vibration, physical shocks, wet, dirt and extremes of temperatures.

No other assembly of electrical units works in more unfavourable circumstances. Yet there still remains a considerable lack *of understanding, and in many cases actual disbelief, as to the practical and! technical reasons for occasionally replacing spark plugs and high-tension cables; with new ones.

In many cases it is the experience of service stations that ear owners fail to keep up the level of the electrolyte in the battery, and it is not rare for a battery to be allowed to become almost bone-dry. The recommended practice of replacing spark plugs with new ones 1 at intervals of 10,000 or 12,000 miles of car travel has remained to be praised by many critics who previously had laughed at the idea as a piece of propaganda by spark plug manufacturers. If plugs do not actually we'ar out through long use, their insulating properties certainly do—very gradually but surely. When the insulation becomes impaired beyond a certain point the electrical leakage between the central electrode and the body of the plug is sufficient to weaken the spark at the business end of the plug.

The advent of the compression engine has attracted attention to the necessity of ignition cables being kept in an electrically efficient condition. With low compressions, almost any kind of ignition wiring was satisfactory, provided that an actual short circuit did not occur.

With higher compression inside the cylinders, there is more electrical resistance for the spark to jump the gap in! the spark plug.

As a result it is necessary for voltages to be much higher, and the insulation of the cables is l subjected to greater electrical stress. When, through heat and the phenomena known as corona, the rubber insulation becomes porous and cracks, the electricity carried in the conductor in the centre of the cable leaks' to the grounded portion of the engine' rather than be directed to the spark' plug gap. A defective cable may cause a weak' spark at the plug points, intermittent 1 misfiring, or a complete breakdown of ignition to the particular cylinder. Apart from the corona effect and heat, there are other causes of'deterioration of the cable. MUD AND CREAM. This is not the foundation of a new slimming diet nor does it relate to, a face pack or other beauty compound. The title has, indeed, been chosen to draw the attention of motorists to the very practical virtues of cream, biscuit, or other light colours as body finishes.

There appears to be a strong belief that for utility cars should be finished in dark colours. There) may be greater fallacies than this theory, but it is doubtful. Light-coloured cars have the merit of not showing dust in the summer and of making the best possible background to mud in the winter. If you d'oubt this an inspection of cars on the road' or when parked will rapidly settle the matter.

Ifi is, however, a good scheme to have black, mudguards. The “other fool” is always liable to bump, these, and matching light colours 1 is admittedly more difficult than black, as; according to the artists, there is only one shade of this.