If the secondary circuit is proven to be in good condition, attention should be paid to the carburetor, for a poor mixture, or water in the gasoline, will cause the engine to miss. A badly fitting exhaust valve will permit the burned gases to be drawn back into the cylinder, the fresh charge thus being weakened, and as the valve will shift around on its seat, this may happen irregularly. Missing will also be caused by a weak or broken inlet-valve spring, or by the sticking of the inlet valve, these conditions resulting in failure to retain the mixture in the combustion space during the compression stroke. This pushing back into the inlet pipe of the charge will usually produce a popping or gurgling noise that is easily recognized.
MISSING AT HIGH SPEED
If an engine runs well at low speed, but misses when speeded up, the trouble may be due to weak battery, stuck vibrator blade, or loose connections. A battery that will produce a good spark at low engine speed may not be able to respond to the greatly increased demands of high speed, and similarly, at low speed the period during which the timer holds the circuit closed is longer than at high speed, and the vibrator has more time to get into action before the circuit is broken. A loose connection may be so shaken by the vibrations of high speed as to break the circuit.
ENGINE STARTS WELL, BUT COMES TO A STOP
If the battery is nearly exhausted, it will recuperate during a rest to such an extent that it will produce good sparks, but as its condition of strength is only temporary, the engine will slow down and come to a stop as the current fails. This action of the engine may also be due to carburetor defects by which the vibrations of running either cause the flooding of the float chamber or the clogging of the spray nozzle, the mixture in both cases becoming noninflammable; the condition will also result from an air-bound supply tank.
OVERHEATING
It sometimes happens that the engine will continue to run after the ignition circuit is opened. This may be due to a failure of the water circulation, which is indicated by the low temperature of the radiator, or by the low speed or stopping of the fan from a slipping or broken belt. If the cooling system is working properly, the lubrication must be investigated. If the oil is flowing as it should, the cause will be found in the formation of a carbon deposit in the combustion space, fine points of which will become incandescent and ignite the mixture as it passes into the cylinder or as it is compressed. A more unlikely fault may be that the points of the spark plug are fine enough to become heated in a similar manner; modern plugs are made with points of such size that this possibility need hardly be considered. This ignition of the mixture by other cause than the proper spark is termed preignition.
ENGINE COMES TO A STOP
If the engine stops suddenly, the fault will be found in the accidental opening of the switch, or the breaking of a wire or connection. An abrupt stop cannot be caused by anything but an interruption of the ignition circuit, for the cessation of the gasoline feed will bring the engine to a stop slowly, the gasoline in the float chamber and spray nozzle being sufficient to permit weakened explosions before the supply entirely fails. A slow stopping of the engine from causes other than those previously mentioned may be laid to defective gasoline feed, the emptying of the tank, the clogging of the supply pipe, float valve, or spray nozzle, or the closing of the supply cock from jolts and vibrations.
If the engine comes to a stop when there appears to be no fault with the explosions, the cooling or lubricating systems may be at fault, resulting in the heating of the cylinder to such an extent that the piston sticks or seizes. Excessive heat is proof of this, the burning or “frying” of excess oil on the outside of the cylinder being an early warning. A seized piston may be freed by injecting kerosene into the cylinder, and cranking, the cylinder first being given time to cool.