What should be done before checking valve clearance or adjusting shims?

Before checking valve clearance or adjusting shims, cooling the engine to room temperature is essential. This procedure is important because engine components expand when heated. If the engine is still warm, the measurements taken for valve clearance may not be accurate, leading to improper adjustments. As the engine cools, valve guides and shims contract, which ensures that you’re measuring the clearance in a state that reflects normal operating conditions.

While removing spark plugs, running the engine, or ensuring it is free of carbon deposits may be relevant tasks during engine maintenance, they are not the primary steps necessary for the precise measurement of valve clearances or shim adjustments. Proper temperature allows for an accurate assessment of the engine's components, making it crucial to start with the engine at a stable, room temperature.