Technician A says that installing a thicker spacer in the high speed poppet valve makes the engine run lean at higher rpm. Technician B says that if a larger main jet is installed to accommodate fuel delivery over 6500 rpm, fuel would run lean at low rpm. Which technician is correct?

• A. Tech A
• B. Tech B
• C. Both
• D. Neither

Answer: (C)

Fuel metering in a carbureted engine is done by several circuits that share air and vacuum signals, and changes to one part can shift the mixture across different RPM ranges. A thicker spacer in the high‑speed poppet valve changes the airflow characteristics and timing of the high‑speed metering path. That alteration can reduce the fuel signal at high rpm, so the engine runs lean as it screams up to and beyond 6500 rpm.

Installing a larger main jet increases the amount of fuel the main circuit can deliver when that circuit is active (typically at higher throttle). If you’re counting on this jetting change to cover fuel needs above a certain RPM, but the idle and pilot circuits aren’t re-tuned accordingly, the overall balance can be upset. During low‑rpm operation, the engine relies on the pilot/idle circuits, and the altered interaction with the main circuit can lead to a lean condition there as well.

So, both changes can produce lean mixtures in their respective ranges due to how the carburetor’s circuits interact and affect the overall air–fuel balance.