A stuck valve on a Continental aircraft engine is one that no longer slides easily in its valve guide. The valve may refuse to open or, once open, may remain stuck in the open position. Both intake and exhaust valves can stick open or closed. Stuck valves are usually caused by a buildup of carbon deposits in the valve guide, especially in the area around any corrosion on the valve stem. Heat is the primary cause of valve sticking. Oil is oxidized in the hot valve guides, and carbon deposits form, especially around any corrosion on the valve stem. Thus, valves are more likely to stick during hot weather than during the cool winter months.
There are several ways to diagnose a stuck valve. Following a run-up of the engine, remove the cowl and drip a little water on each cylinder. The water won't sizzle on a cold cylinder, and you can suspect a stuck valve on that cylinder. Pulling the prop through each of the four or six cylinders is another good indicator: there will be no compression on the cylinder with a valve that is stuck open. Finally, you can remove the rocker covers of a suspect cylinder and, with a thumb on each of the valve ends, rock the valves open and shut. You will feel the valve dragging if the valve guide is clogged with carbon.
If a valve sticks while airborne, the engine may develop a slight roughness, or the roughness can be quite pronounced, with a noticeable loss of engine power. In severe cases, a stuck valve can result in a forced landing. Internally, a stuck valve can wreak havoc on other engine parts. The surface of the camshaft follower can be damaged, or a push rod can bend. In extreme instances, the rocker support can break. The longer an engine operates with a stuck valve, the greater the chance is that significant damage will occur. At the first signs of a sticking valve, an aircraft should be grounded.
One method of dealing with a stuck valve---short of pulling the cylinder for a rebuild or replacement---is the "rope trick," a time-honored procedure among many aircraft mechanics. Remove the top spark plug in the suspect cylinder. Rotate the prop until the piston is fully drawn back into the cylinder. Then feed a length of 1/4-inch rope into the cylinder through the spark plug hole until the space between the face of the piston and the top of the cylinder is filled. Pull the prop through, forcing the piston to compress the rope against the face of the stuck valve. With the rope still compressing the valve spring, remove the rocker arm cover, rocker, valve keepers and valve spring. Remove the rope from the cylinder, and use a piece of safety wire inserted through the spark plug hole to "lasso" the valve behind the valve seat. Lower the valve out of the guide and into the cylinder. Ream the valve guide to clear it of any carbon deposits. Using the safety wire lasso, maneuver the valve stem back into the guide, and reinstall the spring, keepers, rocker and rocker arm cover.
Heat is the primary culprit in valve sticking, aided by carbon deposits from heavily leaded fuels, as well as improper leaning procedures. Using lead scavenger products such as Alcor TCP can help reduce carbon buildups in the valve guides. But keeping engine operations cool with proper leaning, and avoiding steep climb-outs on hot summer days, will probably go as far as any engine additives.
