What indicates when the mixture is moved to cutoff and RPM drops immediately to zero?

• A. Idle mixture set too lean
• B. Idle mixture set too rich
• C. Spark plug fouling
• D. Vapor lock

Answer: (A)

When you move the mixture to cutoff, you’re cutting fuel supply entirely. If the idle mixture is set too lean, there is already too little fuel to sustain combustion at idle. With fuel almost gone, removing any remaining fuel causes the engine to lose its ability to ignite and run, so the RPM drops to zero immediately.

A too-rich idle mixture would still leave excess fuel in the flow, and shutdown might not be so abrupt. Spark plug fouling causes ignition problems and can make shutdown rough or delayed, not an instant cut. Vapor lock interferes with fuel flow in the lines and can cause power loss, but that’s due to fuel delivery issues, not the immediate effect of moving the mixture control to cutoff.