Which of the following is not a reason for valve failure?

The choice of "carboned" as the correct answer reflects an understanding of valve failures. Carbon buildup in an engine can lead to various issues, but it is not primarily identified as a reason for valve failure itself. Instead, carbon buildup may contribute to other problems, such as reduced efficiency or performance, but valves can still function even when carbon is present.

In contrast, worn springs can impede the ability of the valve to close properly, leading to a failure in the valve's functionality. Improper installation of valves can also create significant problems, as misalignment or incorrect seating can prevent them from sealing correctly. Overheating is another serious issue that can warp or damage valves, causing them to fail.

Thus, while carbon buildup affects engine performance, it does not directly lead to the failure of the valves themselves, making it the exception in this context.