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At the end of last season I was riding my motorcycle when suddenly the motor turned off. It was a harsh stop, not dying slowly like when there is no fuel left. Also just the engine stopped quietly, so I assume the cylinder didn't break down (it happened before on another motorcycle and there was a loud bang when it stopped -> piston seizure).

When I try starting it, the electronic starter is turning but the engine won't start at all. Push starting also doesn't work. Also while starting, "air" (I don't know how to check if the fuel actually gets burned) gets pushed out of the exhaust system. So I assume there is enough compression. Haven't checked it though.

The bike is a Yamaha WR125R 2013 Model.

What I already checked:

  • Spark plugs are sparking
  • Fuel is getting injected
  • tried a new CDI
  • push starting doesn't work either
  • battery is full

I didn't want to remove the motor before I can be sure that the motor is the cause. I assume it's just a minor problem that I somehow don't see.

Maybe someone has an idea how I can start enjoying the nice weather in germany :D

  • Did you try a new spark plug? – Moab Apr 12 '18 at 20:35
  • Yes,. I purchsed a brand new one, doesn't work either. – Canonip Apr 13 '18 at 4:30
  • You have the basics, fuel and spark. Does it even pop or sputter? Have you drained the fuel and put in fresh? Possible dirty fuel injector not spraying fuel correctly? Have you verified the ignition timing? How about intake airflow; is it blocked? – CharlieRB Apr 13 '18 at 12:50
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You mention that you have the correct ingredients for combustion as well as engine rotation, apart from one, compression. Check that.

If you don't have a compression tester.. Put the bike in first gear and try to push it. If the rear wheel tries to lock up you likely have compression, however if the rear wheel rotates without issue whilst in gear then you likely don't have any or sufficient compression. And investigation will be needed as to why there is none, such as a blown head gasket, broken piston ring, valve etc.

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