By _timing the valves_ is meant the setting of the cam shaft in such a position that the valves are opened at the correct point in the stroke. It is necessary to time the valves only when the cam shaft has been taken out and must be replaced. The principle of valve timing should be understood, however, in order to be able to tell whether an engine is timed correctly. It will usually be found that the face of the flywheel bears letters and figures that are indicators of the timing of the valves. This arrangement on the E-B engines is shown in Figure 94. Two lines are cut in the face of the flywheel, one marked ex. cl. 1-4, which means exhaust valve closes, cylinders 1 and 4, and the other marked CENTER 1-4, to indicate that the pistons in those cylinders are on center. A straight-edge is held against the finished surface of the housing and the crank shaft is turned to bring one of the marks in line with it; at that point the valves or pistons are as indicated by the lettering. [Illustration: FIG. 94.—VALVE TIMING, USING MARKS ON FLYWHEEL] [Illustration: FIG. 95.—VALVE TIMING] The flywheel is also marked with a dot to indicate the firing point. When the dot is in line with the straight-edge, ignition should occur with the spark control fully advanced. Figure 95 shows the valve arrangement of the same engine, with the exhaust valve just closing; the point of the cam has passed under the lifter or push rod, and has permitted the valve to come to its seat, but is still holding the lifter against the valve stem. To check the valve setting, hold a slip of tissue paper, such as a cigarette paper, in the space between the lifter and the valve stem, while the engine is cranked slowly. While the cam is holding the valve off its seat the paper will be pinched between the lifter and the valve stem and held firmly. At the instant when the paper is freed and can be moved, the valve is seated and the point of the cam is just passing from under; the proper mark on the flywheel should then be in line with the straight-edge. As the cams for all valves are in one piece with the cam shaft, setting one valve sets them all and checking the setting of one checks the setting of all. Before taking out a cam shaft, two adjoining teeth of its gear should be marked with a prick punch or a small cold chisel, and a similar mark should be made on the tooth of the crank shaft gear that comes between them. In replacing the cam shaft it is then necessary only to return the teeth to the same position. Timing gears are usually marked in this way by the manufacturers. CARBON A kerosene lamp that is turned too high gives a dense black smoke that is composed of fine particles of carbon. A piece of paper held in the smoke is quickly covered with a deposit of carbon, commonly called soot, or lamp-black.