MOTORING.

Gisborne Times, Volume LXXIII, Issue 11949, 20 May 1933, Page 11

MOTORING.

SPARKING PLUGS

(PERIOD UF USE

EFFECT OF GAP ON ENGINE PERFORMANCE.

Sparking plugs nowadays are very reliable if they are not usc-d for too long a period. Some manufacturers say 10,000 miles is the useful life of a plug; this a pretty sound rule for most plugs. Although a plug may look all right after this mileage it- cannot be relied upon owing to deterioration of the insulating material.

3lica plugs get frayed and pitted with little holes .which till wi:h carbon and allow a certain amount of leakage. The “porcelain” tyre is liable to get cracked after long use and so allow leakage to occur. These cracks are not always visible, but will cause misfiring, particularly in starting up in damp weather. The best setting for plug gaps varies considerably with different engines and ignition systems. ,A vide gap gives the best spark, but, oi course, it must not be too wide lor-the capacity of the coil or magneto. The compression of the engine, too, plays a big part in determining the best gaps. Coil ignition allows of a bigger gap (0.015 t-o 0.020 in.) than magneto ignition. The latter varies between 0.010 -and 0.15. Plug gaps set too wide will cause “cutting out” when accelerating and, with magneto ignition, hard starting. Pulling on bills is also affected bv too large a plug gap. If the plug's arc too small hard starting can arise owing to the tiny spark not gi'ing enough heat to ignite the mixtiue. Howci' output and petrol consumption arc adversely affected by insufficient

gap. When cleaning plugs always dismantle them and clean the insulation inside and outside the plug. The- central electrode should not be bent on -any account, as this is liable to loosen the insulation or crack the electrode inside the insulation if the plug has been in use for some time a Always bend the outer electrodes to adjust the gap. Tho sparking plugs ill either a fourcylinder or six-cylinder engine should ahvavs be identical in both make and model. One “odd” plug in an engine may cause the latter to run as though the crankshaft or flywheel were badly out of balance; alternatively, or as well, there -may be loss of speed on hills and on the level, liven fho petrol consumption may be increased. The chief difference- between sparking plugs having some or all of the effects mentioned is length- of reach (the distance the electrodes project into tho combustion space). Every sparking plug manufacturer makes plugs that vary in this respect, dor the reason that ho best location for the -spark within the combustion space Varies with different engines. So- it is not merely highly desirable that all four or six plugs of a- “set” should bo of the same make; they should be o the same model, too. Other points of difference between plugs jmay -have ill-effect up an onginc, if an “odd” one, or more, be used, aro the shape, thickness, and design, of electrodes.