Not wanting to state the obvious, but the M12 is a diesel engine. You don't turn it off by turning the key (or pulling out the fuel line). There should be a decompression handle near the controls (T shaped handle). Pull out the handle to kill the engine- then turn off the key.
Just to clarify, diesels are stopped by cutting off the fuel. Some have a handle, some you pull the throttle back past idle, and some have a solenoid valve in the fuel line that is shut when you turn the key to off. It is not good practice to stop a diesel with the decompression control.