Yamaha YBR 125 Fuel Injection System , FI diagnostics .

• Yamaha YBR 125 for sale on Ebay UK 

  Yamaha YBR 125 parts for sale on Ebay UK

Yamaha YBR 125 workshop manuals on Ebay UK

Gemini said

See more Yamaha YBR 125 blog posts

Decided to do a whole post on the FI diagnostics check it here

Gemini said

Safety Message: Fuel systems are under high pressure. When disconnecting fuel lines, wrap a rag around the fitting to catch spray and avoid working near open flames or heat sources. Always wear eye protection when testing injectors or fuel pumps.

Yamaha YBR 125: Fuel Injection (FI) System & Diagnostics

The Electronic Fuel Injection (EFI) system on the YBR 125 is highly reliable, but when it fails, parts can be expensive (a fuel pump can cost over £250!). Use this guide to understand the components and how to test them before you buy.

1. The Fuel Pump: Testing for Life

• The Prime: When you turn the key, you should hear a 2-second "whirring" sound. No sound? The pump or relay is dead.

• Pressure Check: The system should maintain 35 psi at idle.

• DIY Tip: You can test the pump motor by providing 12V directly to the terminals off the bike, but be careful—running the pump "dry" (without fuel) can burn it out quickly.

2. The ECU (The Brain)

The ECU is located under the tank. While it can’t be easily "tested" without a diagnostic tool, you can verify the wiring. Here is the 18-Pin ECU Pin-Out Guide:

Pin Color	Function	Pin Color	Function
Brown	12V Power	Orange/Black	Fuel Injector
Black	Ground	Yellow	Fuel Pump
Orange	Ignition Coil	Green/Red	Engine Temp Sensor
Yellow/Red	Fast Idle Solenoid	Blue/Yellow	Rev Counter Pulse
Red	Magneto Sensor	Yellow/Green	Lean Angle Sensor

3. The Throttle Body Assembly

On the YBR, the throttle body is a "3-in-1" unit. It combines the Throttle Position Sensor (TPS), Inlet Pressure Sensor, and Intake Air Temperature Sensor.

• Common Fault: If the bike hesitates or has a "flat spot" when you twist the throttle, the TPS is usually the culprit.

4. The "Carb to FI" Swap Myth

Many people ask if a Carb engine fits an FI frame. Short answer: No. Not without significant work. You would need to change the flywheel, stator, wiring loom, and fuel tank. It is almost always cheaper to buy an FI engine.

This post will be about the fuel injection system on the Yamaha YBR 125 , common faults , test procedures repair and diagnosis .

by the way i just bought the Yamaha FI diagnostics machine , probably not necessary , but who cares it looks interesting to me , i know nothing about it tbh what its capable of or how to use it so should be fun to learn .

The Yamaha YBR 125 has a built in engine management system , where a series of flashes on the clocks  , engine management light gives out a fault code.

Yamaha YBR 125 Fuel Injection System Components

the main components are shown below , ecu , fuel pump (£270) and fuel pipe ( £98) and the fuel injector / throttle position sensor assembly all photoed below .

there are other components obviously like crank position sensor and flywheel ,idle control  , wiring loom differences etc etc . 

a lot of people ask me if a Yamaha YBR 125 carb engine will fit into an injection model frame but it will not

not unless you change the flywheel-stator  at least  , it might then be possible but im not sure yet , i will eventually try but have not had time to attempt yet , may be other differences as well .

yamaha ybr 125 fuel pump injector pump and level sender unit

buy fuel pumps from ebay uk here  or Amazon uk here ( be quick they go fast )




or cheaper still the internal pump unit on Ebay UK

testing the fuel pump it easy , i usually just provide 12v power to terminals and listen to motor if its off the bike ,  this tell you it operates but does not tell you if its proving enough system pressure , i do this with my

tool i made for 2 stroke injection engine bikes i used to work on , ill add a photo later , its basically just a pressure gauge fitted on a T and in the middle of a spare fuel hose i had ( these fuel pipes cost £90ea from Yamaha so your brave to cut them up lol . )  , anyway fuel pressure should be 35 psi at idle .

                             


you should hear the fuel pump prime the circuit on initial turn of the key ,
running the fuel pump - tank out of fuel can cause damage to the pump .

and testing the resistance values of the fuel level sensor is easy also see below link for expected values
at the empty and full positions .

You can see all the electrical test procedures for the fuel pump and the fuel level sender unit here .      =================================================================

ECU , cannot really be tested unless fitted to a known working bike or swapping it out with a know functioning ECU.
The ECU is located on the frame underneath the tank and is easily removed and refitted to bike .

18 pin fuel injection ecu

check here for cdi / ecu for sale on ebay uk 

Brown - 12Volts

Black/White - 12 Volts
Black - Ground

Red - Magneto Sensor 

Blue - Throttle Body Sensor
Pink/White - Throttle Body Sensor
Black/Blue - Throttle Body Sensor
Brown/White - Throttle Body Sensor

Yellow/Blue - Throttle opening (Variable)

Green/Red - Engine Temperature Sensor
Yellow/Green - Lean Angle Sensor
Orange - Ignition Coil
Yellow/Red - Fast Idle Solenoid
Orange/Black - Fuel Injector
Yellow - Fuel Pump
Blue/Yellow - Rev Counter Pulse
Light Green - Engine Diagnostic Indicator (Amber Light on Instrument Panel)

White/Black - Head Light

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fuel injector , inlet air pressure sensor and throttle position sensor all can be tested via the

Yamaha Fi diagnostics .

check out the posts on how to test these parts below 

injectors , throttle body / position sensor , fast idle solenoid   , crank position sensor .

        the Throttle body sensor combines TPS , Inlet pressure and
                                     Temperature sensors

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