What might cause an engine to sputter while running?

An engine may sputter while running due to various factors that affect its combustion process. One significant cause is the presence of water in the carburetor. Water can disrupt the fuel-air mixture necessary for optimal combustion. When water enters the carburetor, it can cause incomplete burning of the fuel, leading to an erratic engine performance, noticeable as sputtering or misfiring. This issue is particularly critical because water does not combust and can replace fuel in the combustion chamber, compromising the engine's ability to run smoothly.

This contrasts with the other options. A full fuel tank generally provides more fuel to the engine, which should promote smooth operation, not cause sputtering. A clean air filter allows for proper airflow into the engine, which is essential for optimal performance and should not lead to sputtering. Similarly, a properly functioning spark plug ensures that the engine ignites the fuel-air mixture effectively, supporting consistent running rather than sputtering.