Valve Timing.—In timing valves the first step is the establish ment on the flywheel of the points of dead-center of the cranks. The simplest method of marking the dead-centers is to use a steel trammel having both ends pointed. A steel block, with a counter punch mark on the surface, can be inset into the founda tion, being held by lead or cement grouting, Fig. 108. To establish the flywheel position, when the crank is on upper dead center, one of the valve cages can be removed and the distance from the surface of the cylinder head to the piston, when the piston is approximately at upper dead-center, can be measured and a trammel mark made on the flywheel rim. The wheel is then turned on over past dead-center until the piston is again the same distance from the cylinder head. A second mark is placed on the flywheel rim; the bisection of the distance be tween the two trammel marks gives the exact dead-center for the piston in question. A second but not so exact a method is the use of a spirit level on the crank throws; this, of course, is impossible if the throws are not machined accurately on all four sides. The center mark should be placed on the flywheel rim and stamped with the cylinder number, as example 2TC, in dicating top dead-center of No. 2 cylinder. The same procedure is followed on all the cylinders for both top and bottom dead-centers. These center positions being determined, the next step is the checking of the exhaust valves. The engine should be turned over until the piston of the cylinder in question is about 50 degrees from bottom dead-center. A steel tape line can be used to measure the distance on the flywheel rim from the bottom dead-center line to the point of correct exhaust valve opening. Since the timing given is in degrees, the value must be transformed into inches on the fly wheel circle. Presuming the wheel is 8 feet in diameter, the cir cumference being 302 inches, each degree represents, very closely, u-inch. If the proper opening of the exhaust is 42 degrees ahead of bottom dead-center, a distance of 35 inches is set off ahead of the dead-center mark. This is spotted and stamped E20; that is, exhaust opening for No. 2 cylinder. The flywheel should then be turned slowly until the trammel cuts this E20 mark. The exhaust cam rocker should be firmly in contact both with the cam and with the valve stem. To be doubly certain, the adjusting screw on the rocker should be backed off and brought up again until the operator can feel the resistance of the valve spring against the screw. In case, while revolving the wheel, the mark E20 is passed, the valve should not be checked by bringing the wheel back to the mark. Instead, the wheel should be brought back past the mark at least 12 inches and then again turned until the trammel cuts the mark. This
removes all effect of any back lash that might exist in the cam gears. The same process is followed in checking the exhaust closing position. After this is accomplished, the exhaust valve settings of the other cylinders are checked, the flywheel being properly marked for each position. The admission valves should next be gone over in proper sequence, and all points should be indicated on the wheel for future reference.
In event any of the valves fail to check up correctly, the oper ator is confronted with the question as to the method of changing the setting. If the discrepancy is only a few inches, the clearance between the cam and the valve rocker can be changed, bringing the setting back to the stated values. If the valve opens vastly early, or late, the only recourse is the shifting of the cams by the use of an offset key. This condition is encountered in old engines only. In these engines quite often the entire valve layout is timed late. This is attributable to the wear in the cam gear teeth and can be partially corrected by advancing the cam gear one or two teeth.
The average operator, until he is very familiar with engine timing, does well to time the exhaust and admission valves of one cylinder before proceeding to any of the valves of the other cylinders. There is, in this way, little danger of becoming confused as to the proper stroke upon which the valve'should function. The trained operator customarily checks the valves as they come in sequence. For example, the exhaust opening of one cylinder will be set, then the admission closing of a second cylinder will be checked. This reduces the time occupied in going over the valves by at least 75 per cent. The checking and setting of the fuel valves will be taken up in the chapter on fuel valves.
Leaky Valves.—A leaky admission valve is readily detected by placing one's ear close to the intake nipple or screen. If the leak is from a scored seat, a whistling noise will be heard; this whistling increases in volume as the scoring becomes more pronounced.
A leaky exhaust valve is more difficult to detect: The best method is to place the engine in that position where both the admission and exhaust valves are closed; that is, at the beginning of the power stroke. The injection air line valve should be "cracked," allowing a small amount of air to blow into the cylin der through the fuel injection valve. If the exhaust leaks, the air can be heard flowing through the valve. This same pro cedure will also locate admission valve leaks. While the en gine is running, a smoky exhaust an a decrease in power are often due to a leaky exhaust valve.