Practical Hand Book of Gas, Oil and Steam Engines / Stationary, Marine, Traction; Gas Burners, Oil Burners, Etc.; Farm, Traction, Automobile, Locomotive; A simple, practical and comprehensive book on the construction, operation and repair of all kinds of engines. Dealing with the various parts in detail and the various types of engines and also the use of different kinds of fuel.

by John B. Rathbun

The auxiliary exhaust consists of a series of holes drilled or cored through a rib on the cylinder wall, the holes being so situated that they are covered by the piston until it is at the extreme end of its outward stroke. The holes are not uncovered until the burning charge has been expanded and cooled to the greatest extent possible in the cylinder. As soon as the piston uncovers the ports the greater portion of the dead gas escapes instantly to the atmosphere, carrying with them the greater percentage of the heat and flame. The small amount of residual gas that remains is forced out through the exhaust valve in the usual manner, thus no flame ever reaches the exhaust valve.

The use of auxiliary exhaust ports produces a cooler cylinder as the gas passes over the cylinder wall only once, and consequently is in contact with the walls only one-half of the time usual with the ordinary system. The cool cylinder lessens the liability of PREIGNITION and decreases the consumption of cooling water and lubricating oil. Auxiliary exhaust ports are particularly desirable on air cooled engines.

(141) Valves and Compression Leaks—Misfiring.

Owing to the intense heat in the cylinder, and the action of the gases on the valves the seating surfaces become ROUGH and PITTED which causes leakage and loss of compression. Exhaust valves cause the most trouble in this respect as they are surrounded by the hot gases during the exhaust stroke and are much hotter than the inlet valves.

To determine the value of the compression, turn the engine over slowly by hand.

Leaking inlet valves usually are productive of BACK FIRING or EXPLOSIONS IN THE CARBURETOR intake passages, or in the mixing valves, as flame from the cylinder leaks through the valve and fires the fresh gas in the intake.

MISFIRING OR LOUD EXPLOSIONS at the end of the EXHAUST PIPE are indicative of leaky exhaust valves, if the mixture is correct and the ignition system above suspicion. Misfiring caused by leaky exhaust valves is due to combustible mixture escaping from the cylinder to the exhaust pipe and being ignited by the succeeding exhaust of the engine.

If the engine has more than one cylinder, test one cylinder at a time, opening the relief valves on the other cylinders. Now take a wrench and ROTATE the inlet valve on its seat, for it may be that some particles of carbon or dirt have been deposited on surface of the valve seat which prevents the valve from closing properly. Rotating the valve will usually dislodge the deposit.

Try the compression again; if there is no improvement, rotate the exhaust valve on its seat in the same manner, and repeat the test for compression. ROTATING THE VALVES IN THIS MANNER WILL OFTEN MAKE THE REMOVAL OF THE VALVES UNNECESSARY. When the valves are closed the end of the valve stem should NOT be in contact with the PUSH ROD, or cam lever. Suitable CLEARANCE should be allowed between the end of the valve stem and the operating mechanism when the valve is closed; this clearance varies from the thickness of a visiting card on small engines to ⅛ of an inch on the large. If the valve stem is continually in contact with the push rod it cannot seat properly and consequently will leak. Wear on the valve seats and regrinding reduces this clearance, wear on the ends of valve stems and push rods from continuous thumping increases it. Keep the clearance constant and equal to that when the engine was new. On many engines this clearance is adjustable to allow for wear by lock nuts on the ends of the valve stems or push rods.

If the above attempts have proved unsuccessful remove the exhaust valve from the cylinder, if the valve is in a cage, remove the entire cage; this may easily be done on most types of engines. Always remove the exhaust valve first as the inlet valve rarely requires attention. With small engines, and engines having the valves mounted directly in the cylinder head it will be necessary to remove the cylinder head to gain access to the valves. In such a case use care when opening the packed joint between the cylinder and head, to avoid damaging the gasket.