In order to develop full power, an engine must be in good mechanical condition; that is, the bearings must be free without being loose, the gears must run well, the pistons and their rings must not bind or be too free, and so on. It must be properly lubricated and cooled, compression must be correct, it must get a good mixture, and ignition must take place at the right time. If an engine gives trouble, it is because one of these systems is not working properly, and it is not at all difficult to locate the cause and to correct it. If an engine gets a good mixture, which is ignited properly, it will run; if it will not give any explosions it is because one or the other of these systems is not working properly. An inspection or a simple test will show which one is at fault. ENGINE WILL NOT START If an engine will not start after being cranked a dozen or twenty times, it is useless to continue to crank it. It is not getting either a proper mixture or an ignition spark, and it saves time and energy to find out where the trouble is, rather than to keep on cranking in the hope that something may happen. When a tractor engine refuses to start, the trouble is usually with the mixture, and, more often than not, this is due to carelessness or to forgetfulness. The tank may be empty, or the fuel valve may be closed, so that the carburetor is dry; see if there is fuel in the carburetor bowl. The engine may have been shut down while running on kerosene, instead of having been switched to gasoline for the last few minutes of its run, so that the carburetor, intake manifold and cylinders contain kerosene, which will not vaporize without heat, instead of gasoline, which will. In this case the engine must be primed with gasoline. If too much gasoline has been used for priming, the cylinders may contain a mixture that is too rich to ignite; the engine should then be cranked briskly with the fuel shut off and the compression relief cocks open, to clear out the rich mixture and fill the cylinders with air. Water in the fuel will make starting difficult or impossible. It is easy to forget to shut off the water valve of the carburetor when stopping the engine, and when starting, water from this valve will prevent the forming of a mixture and will also interfere with the ignition. If the mixture is apparently all right, the fault may be in the ignition. A drop of liquid fuel or of water, for instance, may be on the spark plug points; this will short-circuit them and no spark will be formed, although the sparking current is passing. If there is a suspicion that the ignition system is at fault, and that the magneto is not producing a sparking current, it should be tested, as explained in Chapter XII. Starting in cold weather is always more difficult than starting when it is warm. Helps in cold weather starting are given in Chapter XI.