With a double carburetor, which has a separate fuel bowl and spray nozzle for each fuel, nothing more is required than the switching of one or the other into action; when the two parts have once been adjusted, they require no further adjustment. Carburetors that use the same spray nozzle for both gasoline and kerosene will require a readjustment when the switch is made, for, as kerosene is thicker than gasoline, it will require a larger opening for a sufficient quantity to pass. This readjustment is a slight opening of the needle valve on switching to kerosene, and an equal closing when gasoline is again used.
A few minutes before the engine is stopped the carburetor should be switched from kerosene to gasoline, so that when it is shut down the fuel bowl will contain gasoline and the cylinders gasoline mixture. This is done to make it possible to start the engine. If the engine is stopped on kerosene, it cannot be started if it has had time to cool. In such a case the fuel bowl must be drained of kerosene and filled with gasoline, and the engine must be cranked until the cylinders receive a clean gasoline mixture.
When an engine is working at full power on kerosene, it gets much hotter than would be the case with a gasoline mixture. Carbon particles in the cylinder, and projecting bits of metal, such as thin spark plug points or the edge of a screw thread, become so hot that they glow, with the result that they ignite the incoming fresh charge and cause preignition. The effect of this is to cause a pounding or knocking that is very noticeable. It is then necessary to use water, which is provided for in the carburetor.
Water has the effect of cooling the intensely heated parts, and only enough should be used to prevent preignition. When the knocking is heard, water should gradually be turned on, using no more than is necessary to stop the noise. Too much water will cause the engine to miss by collecting on the spark plug points, thereby preventing the passing of the ignition spark.
Hard water should not be used, for it will form scale, which will interfere with the action of the carburetor. Only soft water should be used, and preferably rain water.
Whenever the engine is stopped, the fuel valve at the tank should be closed to shut off the carburetor supply. If this is not done, the float valve will be the only thing that prevents the fuel from running out, and should the float valve leak, the fuel will be wasted.
MAGNETO AND IGNITION SYSTEM
A magneto that is kept clean and properly oiled rarely gives trouble, and it is a mistake to blame it whenever the engine runs irregularly or will not start. Its adjustments should be changed only when the other parts of the engine have been proved to be in good condition.
The working parts of a magneto are enclosed, and practically proof against dust. It should be wiped off frequently, and dust and grit should not be allowed to collect around the oil holes, for otherwise it will work into the bearings and damage them.
Dust and dirt are especially injurious to the circuit breaker, which should be frequently inspected and cleaned. Very little oil should be used on it, and this should be the light oil used for typewriters and sewing machines. A thicker oil will become gummy, and will prevent the free action of the lever.