DIAGNOSING TROUBLE

Rotorua Morning Post, Volume 3, Issue 622, 29 August 1933, Page 2

DIAGNOSING TROUBLE

TECHNICAL KNOWLEDGE MERELY LOGICAL REASObttNG. (By L. Mantell in "The Light Car and Cyle.) Most people are apt to regard the rapid and accurate diagnosis of functional trouble as an art or faculty dependent upon an intimate technical knowledge of the parts concerned, Whilst th(is is :. ohVioi^sly helpful, it is equally certain that ordinary logical reasoning, will, in ihe riiajority of cases go a deal farther towards suecessful diagnosis th'an any very intimate knowledge of the construction or functioning of these parts or acces.sories concerned in the breakdown or disorder. tt is remarkatle how very few people can drive "a car, on which, for in^tance, a. bad misfire has occurred, and say quickly and accurately whether it' is an ignition or a carburetter fanlt; yet no special knowledge of either department is necessary to enable an immediate verdict to be given in most cases. In order to appreciate this, consider the various types of misfires, from the complete stall — or, perhaps, refusal to start — to th'e occasion'al interruption in an otherwise rhythmic hum. Why Blame the Plugs. How often does one see, in the first-mentioned ease, the driver descend, open his bonnet and proceed to ■examine or change the plugs ? Yet a moment's thought should make it ahundantly . clear, without any detailed knowledge whatever either of.igI nition or carhur.ation, that a complete sta.ll, or a refusal to start, if due to plugs means that all of them must simultaneously have broken down! Not an utter impossibility, of course, bu't one involving such an amazing freak of coincidence that, excepting in very special circumstances, it can be safely dismissed as impossible. Now what are these special circumstances ? In the case of a complete stoppage while running, even if preceded by spasmodic misfiring, there simply are none, and to examine plugs for the cause of such an occurrence is merely indicative of cr,ass thoughtlessness. (Even to examine them in seeking a cause for refusal to start in the morning after perfeet running the night before is hordering closely upon a similar state of mentality, but, here there can be exceptions. There are probably three in all. Occasionally, during an exceedingly damp night, when moisture condenses on and trickles down stone or non-absorbent walls, it can happen. that small heads of dew will settle not only in the terminals, porcelains .and leads, but also on the actual electrodes and internal surfaces of the insulatoi's. There are probably one or two nights per winter when these conditions occur, and nothing but the removal and drying of the plugs in the oven will coax a start. As to the other two possibilities, a thoughtful reader should almost be1 able now to forecast them in advance himself, bearing always in mind that the circumstances of their occurrence must be those which simultaneously affect all the plugs at once, for no other condition, of course, will meet the case. As everyone knows, certain cheap plugs with single thin electrodes will rapidly burn apart ofter a long and fast ride. In one long and hard day's run it is quite possible to add a full millimetre to the normal clearances when cheap plugs are used with a ho-t spark magneto. While the engine is still warrn and easily turned, all may ibe well, but, after cooling down over night, it may be found utterly impossible to start from cold with the wide clearances bwing to the sticky nature of the engine and"to the tardy evaporation of the cold spray. Setting the points closed will here effect a cure. • The last case is almost on a par. The magneto may, during the course of a day's run, -have developed some progressive disorder causing a considerable weakening of the starting spark. While the engine is still h'ot and the mixture, therefore, readily inflammahle, the trouble may not be spotted, but after cooling down overqight restarting from. cold may be impossible if the plugs gaps are slightly over their normal, and closing' them up a little will enable a start to be made. _ . Strictly speaking, this is not a plug but a magneto wf ault, nevertheless an alteration of their point clearances will permit the- weakeried spark to hridge the gap which, although not normally too great, may now be beyond ihe reduced electrical pressure. Consider now some other- types of misfires, and take first one in which two cylind'ers are affeeted. Here very much logically depends upon which two. Assuming,_ for simplicity, a four-cylinder engine and an ordinary two-pole magneto, it will be evident on consideratiQn that the distribution of the misfires can . help us greatly with the diagnosis. There are two cams on the contaet breaker, oue subplies the spark for cyiinders 1 and 4, and the other for 2 and 3. If, therefore, .the misfire is found •to be consistently in either of these pairs exclusively, suspicipn is immediately thrown on a slightly untrue cam ring or insujfficieiit point separation, which will quickly show up any difference in the cam shapes, hut if the misfire as general over all the plugs the source must clearly be sought for in some part wlych is common on all four, such as the contaet points, the rocker, the condenser, or the central memhers of the high tension distributor. Shorts in the high-tension leads, the fixed part if the distributor or trouble with the sam rings which supply specific plugs rre naturally cut out in general misiring. If, on the contrary, groups 1 and 2 >r 3 and 4 misfire, it is clear that the mntact breaker has nothing to do vith it, for tHe fact o£ the alterhative

group firing regularly shows that hoth of the "breaks" miisi; be correct and the trouble is either in the leads or in the fixed part of the distributor. Plug trouble is, of cours.e, possible, but improbable, because it involves the simiiltaheous disorder of two plugs which would be very coindental. (To be continued.)