MISFIRING

Putaruru Press, Volume VII, Issue 301, 15 August 1929, Page 7

MISFIRING

LOOKING FOR CAUSE. An engine does riot seem to be pulling well. There is an occasional “ pop ” in the silencer, and sometimes the sharp “ crack ” bf a backlire in the carburetter. Listening to an exhaust when the engine is running slowly shows that the firing is not very even. THrire is also present the acrid smell of a rich exhaust mixture. When pulling hard uphill the engine knocks and there is a distinct thump in the engine at certain speeds. Now, what do these symptoms indicate ? There is undoubtedly a case of misfiring. For some reason or other one cylinder” is not firing regularly, ot perhaps not firing at all. But which ?' . An experienced garage hand ‘ will find out the trouble in a few seconds. And why not you? We will suspect a missing plug. Is there a wooden-handled screw- , driver in the kit? If so, hold it by the wooden, and therefore insulated, handle, so that the blade touches ths top of each sparking plug in turn while the point rests on the metal of the engine. Just let the point touch the engine and break the contact slightly; of course, while the engine is running slowly. Now we shall be able to ascertain two things. The first is that if there is a regular spark at thA point of the screw-driver, the ignition current is reaching that particular plug all right. Secondly, that if the engine nuts out or slows Up, possibly very jerkily, when the plug is shortened, then the plug is firing correctly. Proceed with the others. Perhaps one is found where the spark is good hut the shorting of the plug makes no difference. Stop the engine and take the plug out. It will probably be found that the cause of the trouble is a film of oil, possibly burnt up into a solid piece of carbon, across the points of the plug. Clean it thoroughly with a knife, a piece of Tag and a little petrol. You should carry a spare plug to save time when you are on the road. The method outlined of testing for a missing plug is the old-fashioned way. Here is the modern: — At any accessory shop you can buy a plug tester for a few shillings. The tester has a little window through which a reddish glow can be seen when it is held against a plug 'while the engine is running—that is, If the current from the magneto or coil or distributor is passing to the plug. If the current is very feeble, as might be the case with dirty contacts in the distributor, then the glow will Be very faint, and if there is no glow then there is at once an indica-

Lion that the current is not passing at all. If the spark is all right at all plugs some other cause must be looked for, such as gummed-up inlet valve. Generally one can tell this by a wheezing noise. A useful little accessory is sold which is like the doctor’s stethoscope. You place one end to different parts of the engine and listen with the other end to the ear. A piece of wood held between the teeth is equally sensitive.