Yamaha YZF R125 testing fuel injector

   

Yamaha YZF R125 Workshop Manuals on Ebay UK    

If your R125 is experiencing stuttering, poor idling, or a "Check Engine" light (especially on 2019+ models), the fuel injector is a primary suspect. Here is how to diagnose and test it across all generations.

1. Symptoms of a Failing Injector

• Hard Starting: The bike cranks but won't fire.

• Misfiring: Especially under load or when the VVA kicks in (6,600+ RPM).

• Erratic Idle: The bike "hunts" for a steady RPM or stalls at red lights.

• Error Codes: * Code 39: Injector circuit open or shorted.

  • Note: On Gen 4 (2023+) models, check the TFT display for specific diagnostic notifications.

2. The Resistance Test (Multimeter)

The most reliable way to check the health of the injector solenoid is a resistance test.

1. Safety First: Ensure the ignition is OFF.

2. Access: Remove the side fairings and locate the injector atop the throttle body.

3. Disconnect: Unplug the two-pin electrical connector.

4. Measure: Set your multimeter to Ohms (Ω). Touch the probes to the two terminals on the injector.

   • Spec (Gen 1 & 2): Approximately 12.0 Ω at 20°C.

   • Spec (Gen 3 & 4 VVA): Approximately 10.5 – 14.5 Ω.

   • Result: If the reading is "OL" (Open Loop) or 0 Ω, the injector solenoid has failed and the unit must be replaced.

3. Testing for "Spray Pattern" & Clogging

Even if the resistance is correct, the nozzle may be clogged with carbon or old fuel.

• Visual Check: With the injector removed from the throttle body (but connected to the fuel line), briefly pulse the starter.

• The Goal: You should see a fine, conical mist. If it "drips" or shoots a solid stream, it is clogged.

• Cleaning: Do not poke the nozzle with a needle. Use a dedicated ultrasonic cleaner or a pressurized injector cleaning kit.

4. The "VVA Factor" (2019–2026 Models)

On newer bikes, what feels like an injector issue might actually be a VVA Solenoid or High-Lift Cam transition failure.

• If the bike runs perfectly under 6,000 RPM but "cuts out" or bogs specifically when the VVA should engage, check the VVA actuator on the cylinder head before replacing the fuel injector.

5. Fuel Pressure & Pump

If the injector is fine but the bike is lean, check the fuel pump.

• Listen: You should hear the pump prime for 2 seconds when you flick the kill switch to "Run."

• Pressure Spec: Most R125 models require approximately 250 kPa (36 psi) of constant pressure.

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Safety Warning: Fuel is highly flammable. Perform all tests in a well-ventilated area away from open flames or sparks. Always wear eye protection when working with pressurized fuel systems.

An easy test to see if you injector is good is to test the resistance value you can also test with the Yamaha FI diagnostics machine which pluses the injector 5 x to test .

you need a good multi meter - i test here off the bike but on the bike 

is easy enough too .

                       a resistance value of around 12.5 is good

If you need a new fuel injector 

buy here on Ebay UK

or try to clean your old injector first see video