Run the aircraft engine until it reaches normal operating temperatures, then remove the engine cowl. (Don't fly the aircraft if you suspect a stuck valve.) Drip some water onto each cylinder head. If the intake valve has been stuck closed, not allowing fuel into the cylinder to be ignited by a spark plug, the cylinder will be cold, and the water won't sizzle.
Check the compression on each cylinder by pulling the prop through. Be sure the master switch is off and the battery is disconnected before touching the prop. Treat the propeller as if the engine will start any time you move the prop. As you pull the prop through each cycle to raise each piston, you may be able to feel that there is no compression on a cylinder that has a valve stuck open.
Perform a differential compression test if you have an air compressor available. Load up each cylinder with compressed air through a regulator gauge. A cylinder with a valve stuck open will hold little or no pressure.
Remove the rocker cover of a cylinder that you suspect has stuck valve. Place a thumb on the end of each valve stem and push the valves in and out. If the valve is stuck or beginning to stick in its valve guide, it will feel sluggish or it won't move at all.
Fix a stuck valve by cleaning the carbon deposits out of the valve guide. Remove the top spark plug of the cylinder, and rotate the prop to lower the piston. Feed 1/4-inch rope into the open cylinder until it fills the space, and pull the prop through, forcing the rope against the valve faces and compressing the valve springs. Remove the rocker arm cover, rocker, valve keepers and valve spring of the affected valve. Remove the rope and pull the suspect valve into the interior of the cylinder. Ream the valve guide and then reverse the process to reinstall the valve.
