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. If there is much sparking at the platinum points, so that they become corroded and rough, it is an indication that the condenser of the magneto is not operating as it should, for the object of the condenser is to prevent such sparking. The only remedy is to renew the condenser. Rough points will spark more than smooth ones; should they get into this condition, they should be lightly filed with a file of the cut known as “dead smooth.” If this file cannot be obtained, pinch a strip of the finest sand paper—not emery paper—between the points, and draw it gently back and forth, smoothing down first one point and then the other. In smoothing platinum points the greatest care should be taken to make them flat and true to each other. After smoothing the points they should be readjusted so that when they are separated by the cam they are from ¹/₃₂ to ¹/₆₄ inch apart. A distributor made with a carbon brush that slides across the contacts will require wiping off at least once a month. Carbon dust will rub off the brush and collect on the face of the distributor; in the course of time this will cause a short-circuit. The distributor is always made so that it can easily be cleaned.