:::No mystery stuff. Fuel injected engines use an electric fuel pump that's turned off when the ignition is off. Most engines with a carb use a mechanical fuel pump driven by the engine to supply fuel to the float bowl in the carb. From there the fuel is drawn into the engine by vacuum. If there is carbon buildup in the combustion chambers it can remain hot enough to ignite the fuel without an electric spark from the ignition system, much like a diesel uses the simple process of compressing air until it gets hot enough to ignite the diesel fuel.