What could cause excessive stem to guide clearances?

Excessive valve seat width can indeed lead to increased stem-to-guide clearances in an engine. When the valve seat is wider than necessary, it creates a situation where the valve stem does not make optimal contact with the valve guide. As a result, there is potential for more movement or play between the valve stem and the guide. This excessive movement can cause problems, including improper sealing of the valve, increased wear, and potentially leading to engine performance issues.

In contrast, other factors such as carbon build-up and poor lubrication typically affect the overall condition of the engine but do not primarily relate to the specific clearance between the valve stem and guide. While incorrect valve adjustments can lead to various issues, they do not necessarily cause an increase in clearance but rather affect the positioning of the valve during operation. Therefore, the excessive width of the valve seat specifically contributes to the stem-to-guide clearance issue.