A grain of sand in the spray nozzle will choke it, and every precaution should be taken to keep this from happening, as well as the other troubles that dirt brings. The best precaution is to strain the fuel through chamois leather, or, if this is not obtainable, through a very fine metal wire screen. In fuel systems that use a pump, the sticking of the check valves, and the leaking of the pump through poor packing, will cut down the supply of fuel. If air can leak into the carburetor or intake manifold, the proportions of the mixture will be altered. To test for leaks, run the engine, and with a squirt can squirt gasoline on the joints or other places that are suspected of leaking air. If there is a leak, the gasoline can be seen being sucked in. Air must enter the tank to take the place of the fuel that flows out, and this is provided for by a small hole drilled in the tank-filling cap. If this hole becomes stopped up, the fuel will not flow, and the engine will come to a stop. There is a similar hole in the top of the float bowl of most carburetors, and this also must be kept open. An engine is always started on gasoline, for that will form a mixture when it is cold. Before switching to kerosene the engine must be hot, and this will take several minutes of running on gasoline. 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.