MIXTURES.--Too rich a mixture has the effect, in many cases, of causing a deposit of carbon which is bad for the engine. It coats the walls of the cylinders, and is hard to remove. The application of petroleum and alcohol, if allowed to remain in the cylinder for some hours, will aid in taking it out, but removing the cylinder and scraping is the only safe method. The usual way to test the cylinders to see whether either misses fire, is to cut out all of the spark plugs except one, and then test that, and so with all the others in succession, and in this way the location of the trouble will be discovered. SPARK PLUGS.--It is also the case that carbon deposits on the plug points will become heated up to such a point that pre-ignition will take place. Over-heated cylinders may cause this, and in certain cases, where the rotor arm wears, at the contact point, it leaves a trail of metallic particles over which the current will travel. THE WEATHER.--Cold weather is often a serious check to the starting of an engine, the water jacket, or some of the piping may be frozen, or the lubricating oil may become too thick to render proper service. DRAINAGE.--A careful operator will see to it that when the car is left all the water will be drained from the pipes and the water jacket and pump, and the parts can be dried out by running the engine for a minute or so, during the time of draining, so as to heat up the parts. CHAPTER X COOLING SYSTEMS Proper cooling is a necessary feature of all gasoline motors, otherwise the intense heat of the burning fuel would expand the pistons to such an extent as to prevent their free motion in the cylinders, as well as destroy the spark plugs, injure the springs, and make lubrication a difficult matter, if not impossible, by burning up the oil. AIR COOLING.--Cooling was originally obtained by using air, which was blown against the cylinders; but this was not generally developed to a satisfactory degree except for small motors. Air does not take up heat readily, whereas water is the greatest absorbent known, and in the primary stages of the art water was objected to on account of its weight, and for the further reason that the jacketing of the engine was considered a needless expense. One of the best known devices to increase the cooling capacity with air cooling, and now largely used in motorcycles, is to provide the cylinders with a plurality of thin broad ribs, annularly-disposed, as shown in Fig. 72a. [Illustration: Fig. 72a. Increasing Cooling Area.] AIR-COOLING DEVICES.--A highly-heated metallic surface actually repels such a subtile fluid as air, hence it is necessary to supply the cylinders with a blast of air, and also provide a greater cooling area, so that if the ribs themselves can be cooled, the temperature will be decreased in proportion to the enlarged surface thus provided.