MOTORING

King Country Chronicle, Volume XXXIII, Issue 4811, 17 July 1939, Page 3

MOTORING

UNEVEN IDLING. CAUSE OF FAULT. METHODS OF DETECTION. It is quite common for a car to run satisfactorily at all but idling speeds. This is a fault which can be extremely embarrassing in traffic when a motorist, attempting to move off after receiving a point duty constable's signal, finds that his engine has stalled. Unless the engine is demonstrably inefficient at all speeds, this tendency to idle unevenly and stall without warning is rarely caused by anything serious, and usually can be fixed by a garage mechanic in a few minutes. However, for those motorists who like to tinker with their own cars during the week-end, a few hints on the subject may be of some help. First of all, there must always be considered the possibility that the adjustments which control an engine's idling speed may have been altered. These should be checked first. There are two adjustments, one which affects the idling mixture, and the other which sets the minimum throttle opening and, therefore, the minimum speed at which the engine will run.

In the average modern high compression car this throttle opening is set to give a speed of from 6 to 7 m.p.h. in top gear on a level road, and is adjusted by a small screw against the throttle arm on the carburettor. The ilding mixture adjustment is a small knurled screw, set at an angle in the body of the carburettor, and this can be turned left or right until the leanest mixture at which the engine will idle evenly is reached. Turning in the screw makes the mixture leaner, and turning it out makes "it richer.

Water and Dirt. If careful adjustment of these two things does not effect a cure, there remain the carburettor and the ignition to be inspected. Owing to condensation in kerbside tanks and in the cars' own fuel tank, water will collect fairly rapidly in the fuel system, and, as this definitley upsets an engine's running, an examination should be made first of all to see whether there is any present.

In many carburettors, there is small screw in the base of the float bowl., by which the petrol water may be drained away. If the hand is cupped beneath the float bowl to catch what is drained away, water is easily detected as it collects in globules, whereas petrol will immediately spread. When the. carburettor has been drained, it is also as well to drain the glass bowl beneath the petrol pump, which is there to trap foreign matter.

The Ignition.

If all the idling adjustments are correct, the carburettor is clean, and the engine still refuses to idle, check the ignition system. In this regard, the commonest offender is the tungsen points of the make-and-break mechanism, exposed when the bakelite distributor head is lifted. These should be clean and smooth. If they are blackened,. then they will need cleaning with a fine magneto file. It should not be forgotten, however that after long service and several treatments with the file, at least one of the points will be worn right away, so that the plain metal to which it was originally fitted is doing duty in its stead. Frequently, an engine will run reasonably well at high er speeds when the points are in this condition, but will refuse to idle. The condition of the sparking plugs also has a bearing on good idling. If the gaps have been burnt too wide, the engine will run unevenly and tend to stall.'Each plug should be removed in turn and the gap cleaned and reset to the requisite .030 inch. It must also be remembered, however, that plugs tend to break down after 10,000 to 15,000 miles, and the whole trouble may be that they need replacing. About the only other thing which can spoil an engine's idling is the incidence of afr leaks in the joints between the intake manifold and the cylined block. Their presence can be detected by squirting oil round them and watching for bubbles as the engine runs. Tightening the manifold or fitting new gaskets will effect a cure.