Hi, Dave and the usual suspects are:
1. Fuel tank empty.
2. The fuel tank has old dead gas.
3. Fuel tank bottom contaminated with ethanol sludge, dirt, water, rust, etc.
4. Fuel supply valve/petcock turned off.
5. Fouled spark plugs.
6. Engine flooded as a result of overuse of the choke.
7. Vacuum hose to the fuel supply valve/petcock disconnected, broken, cracked, or pinched.
8. Fuel valve/petcock or filter clogged.
9. Fuel line to carburetor or throttle body pinched, kinked or blocked.
10. Carburetor float stuck.
11. Fuel injectors clogged.
12. Fuel injectors stuck open.
13. Quick disconnect check ball stuck.
14. Compression below 75 PSI.
15. A stuck-bent-burnt valve.
16. Severely discharged or a damaged battery should have 12.5 volts or more and be able to pass a proper "LOAD" test if necessary, you may have a cursory reading of 12.5 volts or more but little or zero amperage the battery is faulty and must be replaced, AGM batteries fail in this scenario more so than lead-acid batteries.
17. Check battery terminals for damage or corrosion, check the battery cables at "BOTH" ends for loose, corroded, or broken connectors, "INSIDE" and outside the cable harness, perform connector wiggle test and check cables with an ohmmeter if necessary.
18. Loose or corroded wire connection at the coil or plug between ignition sensor and ECM module.
19. Spark plug cables in bad condition and shorting check for spark leakage in the dark, cable connections loose, or connected to the wrong cylinders.
20. Ignition timing incorrect due to a faulty ignition coil, ignition module or MAP, CMP, CKP, O2, TPS, ETP, IAC sensors.
21. Faulty neutral, clutch, kickstand safety switch.
22. Faulty fuel pump or fuse or relay.
23. A stuck bent or burnt valve.
24. Tilt sensor needs a reset.
25. Security system not disarming alarm needs a reset.
26. Check for engine trouble codes.
27. Faulty run stop switch.
For more information about your issue and valuable "FREE" downloads that you will need for viewing or printing please click on the blue links below. Good luck and have a wonderful day.
yz 125 wont start
YZ 125 wont start
YAMAHA YZ125 LC Owner Service Manual
Btw, I’m available to help over the phone in case u need at https://www.6ya.com/expert/gregg_c0ec1df182c7330e