Yamaha YZF-R125 Service Manual: Fi system

The fuel pump delivers fuel to the fuel injector via the fuel filter. The pressure regulator maintains the fuel pressure that is applied to the fuel injector at only 250 kPa (2.50 kg/cm², 36.3 psi). Accordingly, when the energizing signal from the ECU energizes the fuel injector, the fuel passage opens, causing the fuel to be injected into the intake manifold only during the time the passage remains open. Therefore, the longer the length of time the fuel injector is energized (injection duration), the greater the volume of fuel that is supplied. Conversely, the shorter the length of time the fuel injector is energized (injection duration), the lesser the volume of fuel that is supplied.

The injection duration and the injection timing are controlled by the ECU. Signals that are input from the throttle position sensor, crankshaft position sensor, intake air pressure sensor, intake air temperature sensor, lean angle sensor and coolant temperature sensor enable the ECU to determine the injection duration. The injection timing is determined through the signals from the crankshaft position sensor. As a result, the volume of fuel that is required by the engine can be supplied at all times in accordance with the driving conditions.

Fi system


  1. Fuel pump
  2. Fuel injector
  3. Ignition coil
  4. Coolant temperature sensor
  5. ECU (engine control unit)
  6. Lean angle sensor
  7. Crankshaft position sensor
  8. FID (fast idle solenoid)
  9. Air filter case
  10. Throttle body
  11. Throttle body sensor assembly
  12. Intake air temperature sensor
  13. Throttle position sensor
  14. Intake air pressure sensor
  1. Fuel system
  2. Air system
  3. Control system
    Outline of the fi system
    The main function of a fuel supply system is to provide fuel to the combustion chamber at the optimum air-fuel ratio in accordance with the engine operating conditions and the atmospheric tempera ...

    Multi-function display
    1. Multi-function display 2. "RESET/SELECT" button The multi-function display is equipped with the following: a speedometer (which shows the riding speed) an odometer (which shows the t ...

    Other materials:

    Checking the crankshaft position sensor
    1. Disconnect: Crankshaft position sensor coupler (from the wire harness) 2. Check: Crankshaft position sensor resistance Out of specification  Replace the crankshaft position sensor/stator assembly. a. Connect the pocket tester ( × 100) to the crankshaft position sensor c ...

    Checking the front brake disc
    1. Remove: Front wheel Refer to "FRONT WHEEL" on page 4-6. 2. Check: Brake disc Damage/galling Replace. 3. Measure: Brake disc deflection Out of specification Correct the brake disc deflection or replace the brake disc. a. Place the vehicle on a suitable stand ...

    Checking the crankshaft
    1. Measure: Crankshaft runout Out of specification Replace the crankshaft, bearing or both. NOTE: Turn the crankshaft slowly. 2. Measure: Big end side clearance Out of specification Replace the crankshaft. 3. Measure: Crankshaft width Out of specification&nbs ...

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