Motor-car principles; the gasoline automobile

by Roger B. Whitman

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.

THE WIRING

Fig. 50.—Jump Spark Wiring Diagrams.

In laying the wires for the ignition system, only the best wire should be used, and all contacts should be bright and clean. Connections should be tight, and covered with vaseline to prevent corrosion, additional protection being secured by binding all exposed places with so-called electric tape. There should be the fullest protection against chafing and moisture, and no sagging of the wires. Beginning with the battery, the cells should be placed in a box that holds them securely, so that there may be no jolting, for this in working the wires will tend to loosen the connections. In connecting the cells, the wires should be cut to correct length, but not so short that they will be drawn tight when connected. The insulation should be cut off cleanly, and the paraffin that will cling to the metal scraped off. The ground wire may then be run, after which the wires from the two batteries may be connected to the switch. (Fig. 50). On multicylinder engines, one circuit should be completed and tested before another is started, as this obviates the danger of confusing the wires. The primary wire should be run from a coil unit to its timer contact, making sure by the buzzing of the vibrator that the current flows when the switch is closed. A buzz of the vibrator when the timer is not making contact indicates a short circuit. The primary circuit for the coil having been correctly arranged, the secondary connection should be made and tested before laying the primary of the next coil unit. In making the timer contacts of a four-cylinder engine the firing order as established by the opening of the exhaust valves must be kept in mind. With the spark control lever in the retarded position, the timer should be set so that it is just beginning to make a contact when the piston in cylinder No. 1 is at top dead center of the compression stroke. This contact should then be connected to the primary terminal of the first coil, and the secondary of that coil connected to the spark plug. If the firing order is 1, 2, 4, 3, (Fig. 51), cylinder No. 2 will be the next to fire, and the primary of coil 2 should be connected to the next timer contact in the direction of rotation. As the next cylinder to fire will be No. 4, the timer contact that will next be reached should be connected to coil No. 4, the remaining contact point being connected to coil No. 2. In the diagram the timer is supposed to be moving in the same direction as the hands of a watch.

Fig. 51.—Jump Spark Wiring Diagrams.