(f) Defective sparking plug or plugs. This may arise from water or oil or dirt between the plug points; or from faulty insulation in the body of the plug. To test whether the plugs are at fault an easy method is to take a screwdriver with a wooden handle and place the metal blade on the terminal of the plug, letting the point come about one thirty-second of an inch from the metal of the cylinder or any of the pipes; when the engine is turned by hand the spark will be seen to pass across this improvised gap if the magneto and leads are in order.
(g) Defective electrical connexions.
The high tension cables may be broken, or disconnected, or short-circuited. The earth wire may be short-circuited (i.e., in electrical contact with some other wire or metal fitting). There may be a short-circuit in the ignition switch.
(h) Defective magneto or coil.
The low tension contact breaker lever may be jammed so that the make and break is inoperative, or one of the carbon brushes may have got broken. Occasionally one finds the magnets of the machine have lost their power; or there is some electrical defect in the armature or condenser. The battery may have become exhausted. The trembler blade may be stuck up. Water may have found its way on to the high tension electrode or into the safety spark gap.
2. Engine starts up fairly well, runs a little, and then stops.
Take care to notice the manner in which the engine runs and stops. Note whether it runs regularly or irregularly and for how long a time.
If the engine runs regularly with all cylinders firing, then probably the exhaust is choked or the petrol supply fails. Failure of the petrol supply may be due to the use of too small a jet in the carburettor, too low a level in the float chamber, or to partial stoppage in the pipe line. Another cause of this trouble of intermittent running would sometimes be loss of battery power when using coil ignition, i.e., batteries want recharging.
If the engine runs irregularly the trouble is probably due to too much oil in the cylinders causing the plugs to misfire, the presence of water or dirt in the petrol, a defective valve, a broken carbon brush, or poor electrical contact somewhere in the magneto, the low tension contact breaker (coil), or high tension distributor (coil).
To ascertain whether the engine is firing regularly on all cylinders, or to detect which cylinder is misfiring, the best procedure is to open the compression taps in turn while the engine is running and in each case speed up the engine while you have the tap open. Cylinders which are firing well give a sharp cracking noise, those which are not firing merely give a hissing noise. If no compression taps are provided, each plug must be short-circuited to the frame in turn by the screwdriver method given above. The short-circuiting process causes a reduction in engine speed except on that plug which is already not firing. The method is not so good as the compression tap process, because the plugs often get oiled up during the short-circuiting process and the difficulty is accentuated.