Yamaha YZF R125 fuel injection system

   

Yamaha YZF R125 Workshop Manuals on Ebay UK 

The Yamaha YBR 125 was one of the first mass-market 125cc commuters to ditch the carburetor for a reliable Fuel Injection (FI) system. While generally bulletproof, high mileage or poor fuel quality can lead to "surging" or "stalling" issues.

1. How the System Works

The YBR uses a "Closed Loop" system. The ECU (Engine Control Unit) takes data from several sensors to calculate the exact millisecond of fuel to spray:

• IATS (Intake Air Temperature Sensor): Adjusts for cold mornings vs. hot afternoons.

• MAP (Manifold Absolute Pressure): Senses engine load.

• O2 Sensor (Lambda): Located in the exhaust, it checks if the bike is running too rich or lean.

2. Identifying Faults (MIL Blink Codes)

If your "Engine" light is flashing on the dash, the ECU is telling you exactly what is wrong. Count the Long flashes (10 units) and Short flashes (1 unit):

• 1 long, 2 short (Code 12): Crankshaft position sensor (No signal).

• 1 long, 5 short (Code 15): Throttle position sensor (Open or short circuit).

• 2 long, 1 short (Code 21): Coolant/Engine temp sensor.

• 3 long, 9 short (Code 39): Injector circuit (Check wiring or the injector itself).

3. The "Fuel Pump" Issue

A common YBR 125/YS125 quirk is the fuel pump failing in extreme heat.

• Test: Turn the key. If you don't hear a high-pitched "whirr" for 2 seconds, the pump isn't priming.

• Fix: Often caused by a clogged internal filter or a worn motor. On older YBRs, the pump can sometimes "stick" if the bike sits in direct sunlight for hours.

4. Cleaning the Throttle Body

Unlike a carburetor, you don't "tune" the FI, but you do need to clean it. Carbon buildup around the Idle Air Control (IAC) valve can cause the bike to stall when coming to a stop.

• Use a dedicated EFI cleaner spray.

• Avoid poking the butterfly valve with hard tools, as this can ruin the factory calibration.

5. Modern Connectivity (For Newer Models)

If you are working on a 2021+ Euro 5 variant, many mechanics now use a standard OBD2 scanner with a Yamaha-specific adapter cable (3-pin or 4-pin) to read live data. This allows you to see real-time throttle position and O2 sensor voltages.

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Safety Warning: Working with the fuel system involves pressurized gasoline. Never smoke or work near a pilot light (like a water heater). Always wear protective gloves and eyewear.

Fuel is delivered via fuel pump and pressure regulator through a fuel filter and into the fuel injector. The ECU uses the pick up coil / crank sensor to time the fuel injector to open and the other sensors are used with the ecu fuel map tp control the injector on duration. 

The injection duration uses a fuel map inside the ECU and inputs from the throttle position sensor, Intake air pressure and temp sensors , coolant temp sensor and even the lean sensor will cut off fuel if activated..


The fuel injection system on the YZF R125 is straight forward and easy to work on

it has inbuilt diagnostics and you can also use the Yamaha FI diagnostic tool for greater problem solving power.  

                             R125    FI wiring diagram 

R125 fuel pump wiring diagram 

See link here for more info on FI tool 

The fuel pump

you can test this by adding power to the terminals and listening to sound , or turn key and here pump prime , i like to test the fuel pressure too .

i use a home made tool to do this  

CDI obviously times the fuel injectors timing and opening duration 

these can go faulty but test all obvious first buy here on Ebay UK if needed 

Throttle body with other input sensors you test all these components 

if you need these parts buy from here on Ebay UK 

Other posts of interest .

Yamaha YZF R125 testing injector


Yamaha YZF R125 , Yamaha FI diagnostics Tool Scanner Machine


 Testing and cleaning YZF R125 fuel injectors