Hi, Ian before testing any electrical component in the Starter Circuit it is "IMPERATIVE" that you have a fully charged battery of 12.5 volts or more and be able to pass a proper "LOAD" test because your battery may have 12.5 volts or more but little or zero amps causing the battery to be faulty and must be replaced, especially "AGM" batteries.
1. Spark plugs in bad condition or have an improper gap or are partially fouled.
2. Spark plug cables in bad condition.
3. Loose wire/cable connection at one of the battery terminals, especially the "NEGATIVE" cable, check it at both ends, coil, or plug between ECM or ignition module.
4. Throttle controls not adjusted correctly.
5. Water or dirt in the fuel system.
6. Water or dirt in the carburetor.
7. Intake air leak.
8. Fuel tank vent hose and vapor valve plugged, or fuel line closed off, restricting fuel flow.
9. Enrichener valve inoperative.
10. Engine lubricant to heavy (winter operation).
11. Ignition is not functioning properly (possible sensor failure).
12. Faulty ignition coil.
13. Valves sticking.
For more information about your issue and valuable "FREE" downloads that you will need please click on the blue links below. Good luck and have a wonderful day.
Yamaha YBR 125 won start
Yamaha YBR 125 will not start
Yamaha YBR125ED 2005 Service Manual
OEM parts for Yamaha
YAMAHA YBR125ED Owner Manual
Btw, I’m available to help over the phone in case u need at https://www.6ya.com/expert/gregg_c0ec1df182c7330e