A fuel injection controlling system includes an accelerator manipulator. An accelerator manipulated variable detection sensor is configured to detect a manipulated variable of the accelerator manipulator. A rotation speed sensor is configured to detect a rotation speed of an engine. A throttle valve drive motor is configured to open and close the throttle valve in response to the manipulated variable. A controller is configured to control the throttle valve drive motor and to compute basic injection time (Ti) of fuel. A memory is configured to memorize, as a carbon adhesion judgment value (IXREF), a throttle opening degree (θTH) at a moment when a rotation speed (Ne) detected with the rotation speed sensor reaches a target idle rotation speed (NeIdle) by increasing the throttle opening degree (θTH). A corrector is configured to correct Ti with IXREF to correct an air-fuel ratio toward a leaner side as IXREF increases.