Morning sickness is a warning that should be heeded immediately. Flying an airplane whose engine exhibits morning sickness increases the risk of serious engine damage and possibly in-flight engine failure. Hence, the aircraft should be downed for maintenance at the first hint of valve sticking. Valve sticking is influenced both by the design of the engine and the environment in which it is operated. Lycomings have more valve sticking problem than Continentals.
Valves are more likely to stick in hot summer weather than in cold winter months.
The use of heavily-leaded fuels and inadequate leaning can lead to valve sticking, as can infrequent oil changes. Heat is the primary cause of valve sticking. High temperatures in the exhaust valve guide oxidizes oil and forms carbon deposits on the valve guide, and these deposits can cause the valve to stick. The most frequent reason for elevated valve temperatures is valve leakage. All of the combustion gas must pass around the valve face as it goes out the exhaust port.