SETTING THE VALVES
In setting or timing the valves of a gasoline engine, the point to be considered is the closing of the exhaust valve, for upon this good results depend. If this valve is held open too long, the burned gases driven out will be drawn back into the cylinder, and if it closes too soon the greatest possible quantity of burned gases will not have been expelled. The many experiments carried on by the manufacturers, and the attention that they must pay to this point, result in the delivery of cars with valves correctly timed, and usually with marks made on the two-to-one gears to guide in resetting them. In the absence of these guides, the setting of valves need not be difficult, although experimenting is required to secure the best results. The first step is to locate the position of the piston in the cylinder. There is always an opening in the cylinder head—a relief valve, the spark-plug opening, or other—and a stiff wire may be dropped through it, with its lower end resting on the piston and its upper end projecting. As the piston is moved by cranking the engine, the wire will move with it, and is to be marked with a file at its highest and lowest positions. While no fixed rule can be laid down, it may be said that in general the exhaust valve should close when the piston has made from 1/64 to 1/32 inch of its outward stroke. The two-to-one gears having been unmeshed, the cam shaft may be revolved in its bearings by hand; cranking slowly, move the piston down from its highest point to that at which the exhaust should close, and hold it there. Revolve the cam shaft until the nose of the cam is passing from under the roller of the valve-lifter rod, and the valve just closed. This point may be accurately ascertained by placing a strip of thin paper between the valve-lifter rod and valve stem; when the cam is acting on the valve-lifter rod, the paper will be pinched, but the seating of the valve will release it. When the paper can be pulled out, mesh the two-to-one gears, and the relations thus established between the crank and the cam shafts will be maintained. Cranking the engine a few times, using the strip of paper, will verify results, and the engine may then be started and the effect noted. If the running is not satisfactory, unmesh the gears, and mesh them with a difference of one tooth, first one way and then the other, noting results, and retaining the most satisfactory position. Cams are often cut in one piece with the cam shaft, and the gear keyed on the end; there is therefore no chance to make a finer adjustment than what is permitted by shifting the gears one tooth at a time. On multicylinder engines, one cam shaft operates all of the exhaust valves, and the setting of one valve sets all.
When inlet valves are of the mechanically operated type, but controlled by a separate cam shaft, this must be set in a similar manner. The nose of the cam should just be coming into contact with the push rod roller at the instant when the exhaust valve is completely closed. This should occur as the piston begins to move outward on the inlet stroke, the exact position being determined by experiment.
When the cams are so badly worn that if they are set to open correctly they close too soon, the best remedy is a set of new cams; for while the brazing of a strip of brass to the sides of the cams can be resorted to, the result is only temporary at best, and not as accurate as that secured by the use of new cams.
While the tension of the spring of an automatic valve is sometimes controlled by a nut, its adjustment usually depends on the stretching of the spring to strengthen it, or the cutting off of part of a turn to weaken it. The tension should be adjusted, one cylinder at a time, until the best results from each are secured.