Installing a thicker spacer in the high-speed poppet valve is expected to cause the engine to run lean at higher rpm. Which option best describes this effect?

• A. Richer at higher rpm
• B. Lean at higher rpm
• C. No change at higher rpm
• D. Overheats

Answer: (D)

A thicker spacer in the high-speed poppet valve tends to lean out the air–fuel mixture at higher rpm. When the mixture is lean, the combustion temperature rises because there is less fuel to absorb heat during combustion. At high rpm, the engine is already generating more heat, so this hotter burn drives temperatures up faster. If the engine can’t shed that extra heat, it will overheat. So overheating is the best description of the overall effect: the lean condition created at high rpm leads to higher combustion temperatures that can push the engine into overheating.