All fuel oils and lubricating oils contain carbon. When these oils burn in the cylinder, they produce carbon, much of which passes out of the exhaust, while the rest deposits on the valves and on all parts of the combustion space. This deposit hardens, and eventually makes trouble through causing preignition. The deposit is rough, and the heat in the cylinder is sufficient to make the outstanding particles glow; they ignite the incoming charge, and cause preignition. The sign of carbon trouble is a sharp knocking in the cylinder, especially when the engine is under a heavy load. The sound is the same as that caused by too great an advance of the spark. Carbon deposit can be greatly reduced by pouring a few tablespoonfuls of kerosene into each cylinder and cranking for a few turns to spread it to all parts of the combustion space. This will soften the carbon and much of it will be blown out when the engine is next started. Best results will be obtained if the kerosene is poured in after a run, when the engine is hot. If the carbon deposit is too hard to be softened by kerosene, it can be removed by scraping. This requires the cylinder head to be taken off, when the deposit can be scraped and chipped with a screwdriver. Care should be taken to keep the carbon crumbs from getting into the cylinders, valve stem guides, or other places where it would cause wear. In taking off the cylinder head the gasket should be handled carefully, and protected from denting and bending. A battered or bent gasket is a sure cause of compression leaks. In replacing a metal gasket, give it a coat of cylinder oil on both sides to improve its seating. When replacing the cylinder head, set all of the bolts up a little at a time, instead of screwing some of them tight while others are loose. One bolt drawn tight may tilt the cylinder head slightly, and there will be a distortion when another bolt is tightened. This is avoided by setting up all of the bolts a little at a time. Running on too rich a mixture, giving the engine too much oil, and not using an air cleaner in dusty work will carbonize an engine rapidly. Blue smoke at the exhaust is a sign that too much lubricating oil is being used; black smoke indicates too rich a mixture. Carbonizing can be greatly reduced by careful adjustment of the lubricator and carburetor. CHAPTER XIII LOCATING TROUBLE There are many ways in which an engine can give trouble, but these are not serious to an operator who understands the action of an engine, and who works with his brain as well as with his hands. Each of these troubles has a distinct cause; proper care will avoid them, but if they come the reasons for them can be determined by simple tests.