What problems can occur if the air-fuel control unit on a reciprocating engine is not adjusted correctly?

• A. The engine may overheat and seize.
• B. The engine may not reach full power, run rough, or die at low power settings due to incorrect air-fuel ratios.
• C. The engine will have no noticeable issues.
• D. The aircraft will exceed noise limits.

Answer: (B)

The air-fuel control unit sets the fuel amount to match the air entering the engine, so the mixture stays at an appropriate ratio for current operating conditions. When this adjustment is off, the mixture can become too rich or too lean for the throttle setting you’re using. At low power settings, the engine depends on a correct, stable mixture to idle smoothly and to continue running without hiccups. If the adjustment is off, the engine will often run rough, misfire, or fail to hold power, and it may die when you try to pull back to a low power setting or when you ask for full power later. In short, improper adjustment disrupts the air-fuel balance, leading to reduced power, rough operation, and instability at low throttle.