* Ignition Coil: The ignition coil receives 12V from the battery through the ignition switch. When the key is turned, the coil charges up and then discharges a high-voltage pulse to the spark plugs. Turning the key to the "off" position cuts this power.
* Spark Plugs: Without the high-voltage pulse from the ignition coil, the spark plugs can't ignite the air-fuel mixture in the cylinders.
* No Combustion, No Power: Without the combustion of the air-fuel mixture, the engine stops running. The carburetor is still there, supplying the fuel, but the ignition is missing, preventing the engine from continuing to operate.
In short, the carburetor continues to do its job (providing fuel), but the lack of a spark from the ignition system, controlled by the ignition switch, prevents the combustion necessary to keep the engine running. Turning the key off breaks the electrical circuit to the ignition system.
