Fig. 94.—Valve Timing, Using Marks on Flywheel
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.