Pressure-actuated starting valves sometimes fall to operate because of broken or weak valve return springs. Replacement is generally the only solution to this condition; however, some valves are constructed with a means of adjusting spring tension. In such valves, increasing the spring tension may eliminate the trouble. Occasionally the actuating pressure of a valve will not release, and the valve will stick open or be sluggish in closing. The cause is usually clogged or restricted air passages. Combustion gases will enter the air passageways, burning the valve surfaces. These burned surfaces usually must be reconditioned before they will maintain a tight seal. Keeping the air passages open will eliminate extra maintenance work on the valve surfaces.

MECHANICAL LIFT VALVES.The mechanical lift-type air starting valve is subject to leakage which, in general, is caused when the valve sticks open. Any air starting valve that sticks or leaks creates a condition that makes an engine hard to start. If the leakage in the air starting valve is excessive, the loss in pressure may prevent the engine from starting.

Leakage in this type of valve can be caused by an overtightened packing nut. The packing nut is sometimes overtightened to stop minor leaks around the valve stem when starting pressure is applied, but overtightening may prevent the air valve from seating. As in the pressure-actuated valve, there may not be enough return spring tension to return the valve to the valve seat after admitting the air charge.

Obstructions such as particles of carbon between the valve and valve seat will hold the valve open, permitting combustion gases to pass. A valve stem bent by careless handling during installation may also prevent a valve from closing properly.

If a valve hangs open for any of these reasons, hot combustion gases will leak past the valve and valve seat. The gases burn the valve and seat and may cause a leak between these two surfaces even though the original causes of the sticking are eliminated.

Completely disassemble and inspect a leaking valve. It is subject to a resinous deposit similar to that found in a pressure-actuated air valve. Use a specified cleaning compound to remove the deposit. Be sure the valve stem is not bent. Check the valve and valve seat surfaces carefully. Eliminate scoring or discoloration by lapping with a fine lapping compound. You may use jewelers rouge or talcum powder with fuel oil for lapping.

From the preceding discussion, you have learned that the air starting system may be the source of many troubles that will prevent an engine from cranking even though it can be barred over. You will avoid a few of these troubles by following prestarting and starting instructions. One such instruction, sometimes overlooked, is that of opening the valve in the air line. Obviously, with this valve closed the engine will not crank. Recheck the instructions for such oversights as a closed valve, an empty air storage receiver, or an engaged jacking gear before starting any disassembly.