Modern Motoring

Northern Advocate, 14 January 1939, Page 6

Modern Motoring

Be et Good Mechanic As Well As a Good Driver

'there is no need to-day for a. motorist to be a skilful mechanic, but there Is an advantage in knowing one of two elementary things about a car engine, particularly if the owner frequently undertakes long journeys in the country. tor example, misfiring and detonation are nearly always due to very elementary and simple causes, bilk how many drivers who have turned to motoring within last few years can attempt to cure one or the other with a reasonable hope of success? The most common cause of misfiring is a faulty sparking plug. When a car has exceeded 10,000 milestone or more of the plugs is likely to break down in service. Even if They do not fail completely, the chances ara that they will fail partially or intermittently, and many a sluggish and irregular engine has been restored to something., like its original condition by the sitnple expedient of. replacing the whole set. _ ;7vV■ „" • ■ The Faulty Plug. . But we are dealing at the moment with a regular and complete misfire on of the cylinders, and the chances are about 10 to one in favour of the completely broken down sparking plug.' Hence, at least two spares should always be carried in the car. Discovering the faulty plug is a simple matter, but possibly unknown to some of the more recent motorists. It consists of short-circuiting the eefttral electrode of each plug in turn against the body of the engine with a screwdriver, and the faulty plug is found when this treatment does not make any further difference to the running of the engine.

The screwdriver must have a wooden or bakelite handle, and care must be observed that the metal part of the Screwdriver is not touched with the fingers. Otherwise, the amateur mechanic will receive the full benefit of the high tension current. / ■■■•'' , ' • . 1 , .Effect of Kep:a,cemen:. Replacement Will effect a more or less permanent cure if the engine is ih good order, but if that same cylinder soon starts misfiring again more than likely thb plug is “oiling up,” due to wear in the cylinder bore or a bx-oken piston ring, and rectification of that is beyond the scope of the amateur. On the other hand, it is barely possible- that the unevenness is caused by a, tappet adjusted so tightly that it rides against the base of the valve Steffi,. and It ’ will be worth while examining both tappets* of that cylinder to make surd that there is a certain

amount of play between tappets and valve stems when both valves are in the closed position. About the only other thing which can cause the trouble, is a fault in the high tension lead running io the plug—a break in the insulation which allows the current to earth itself against the cylinder block. Detonation. Detonation is the actual explosion of the mixture in the combustion chamber instead of* the extremely rapid burning which should take place. Its occurrence in an engine which formerly has behaved well nearly always indicates an excess of carbon deposits. These have the effect of restricting the available space for combustion, and thus increasing the compression pressures beybnd the points so carefully worked out by the designer so that the mixture explodes on ignition. A top overhaul at the local service .station is indicated. It may, however be caused by using a low-grade fuel with the ignition too far advanced, and experiments may be made with the little adjustment fitted at the side of the ignition distributor head on most cars o fto-day. This is expressly provided for fitting a high compression moderp engine to the Use of both low £nd high octane value fuels.

On the contrary, this adjustment sometimes slips back to the fully retarded position, so that there is a very mysterious loss of power. In such cases it is the first thing that should be 'examined. y. f - , Frequent Pre-Ignition, A phenomenon sometimes encountered and' having a groat deal to do with detonation is pre-ignition, in which ajr engine continues to run for a few revolutions after thd ignition has been switched off. This is caused by some small projection within a cylinder becoming incandescent; under the heat of combustion, and usurping the function of the electric spark by firing the mixture before the proper moment and continuing to do so after the engine has been switched off. Sometimes a small propecting piece of carbon becomes red hot, and is responsible. In other cases the wrong type of sparking plug for that particular engine is fitted, the ' electrodes becoming incandescent. The plugs may rightly be suspected if the electrodes burn away in service far more rapidly than seems reasonable. A “colder” type of plug, that is, one with a greater resistance to heat, should be fitted.

Hints, Information and Advice for Owners and Drivers

Finally, intermittent misfiring may bo caused by blackened and pitted contact breaker points, exposed when the bakelite cover of the distributor is lifted. They may be cleaned with a fine magneto file, available from any accessory store for a few pence.

If the owner has occasion to replace a set of sparking plugs he must be careful not to mix the high tension leads running to them from, the distributor. If he does, fireworks will result.