Runs But Doesn't Start?

Discussion in 'YZF R125 General Discussions' started by Luke Saunders, Sep 2, 2017.

  1. Hi guys,

    Was on the forums not long ago after my engine blew up and I had to do an engine swap but naturally like all 125s, it wasn't long before I ran into a load of problems.

    I've fixed every problem so far but my current one is a absolute myth!

    So basically when pressing the ignition the bike just turns over forever without sparking. Occasionally whilst doing this I'll hear a single engine fire, then maybe another single engine fire 10 seconds later and so on, until eventually; after turning it over for a good 30 seconds, it'll go fire once then quickly after and then after the third fire the engine fully comes alive and revs up to 8000rpm easily no problems.

    What puzzles me though is that when it does start the bike runs flawlessly no problems. So what on earth is stopping it starting first time?!?!

    Spark Plug - checked, strong, consistent spark and have now replaced with brand new one.

    Fuel Pump - Working well, pumps fuel to injector.

    Injector - Working well, sprays fine mist of fuel into combustion chamber.

    So you can probably understand my frustration now as I have compression, fuel and a spark yet it does not start.

    What's further interesting is when I take my spark plug out it's drenched in fuel.

    On the odd occasion it truly does not want to start, and if i have to turn it over for a minute or so I unscrew spark plug and screw it back in which also completely fixes the issues and allows it to start first time, although this is becoming less and less effective.

    Please help me guys :(!!

  2. Just an extra note: I've just DOUBLE checked compression, spark, and fuel which are all 100% working. My suspicion is it could be the ignition timing? If it isn't sparking at the right time that may be what's causing it?
    - that being said; if the timing was out surely it wouldn't run fine once it is running? I thought maybe HT Lead but if that was faulty it wouldn't run flawlessly consistently... In fact if any parts are faulty, it wouldn't run surely?

    Are there any parts which only handle ignition?
    and also is it my engine failing to get from ignition mode into 'cycle' mode?


  3. Blazer

    Blazer Member

    Could it be that you forgot too re connect the engine earth?, It's easy to forget. Other than that, if compression is ok and the fuel and ignition elements are ok, then the only element left would be oxygen/air. Have you checked if the throttle valve is opening, if the air box has any liquid in it, or weather the air filter is blocked .
    Last edited: Sep 4, 2017
    Luke Saunders likes this.

  4. Thanks for your reply Blazer!
    Yeah my suspicions were starting to head for air/fuel... air filter seems fine, I drained the small amount of liquid yesterday but could be tubing in and around air filter... Later I'll upload a picture of a tube which literally goes from the airbox intake to nowhere... maybe I've got something connected wrong? I'll check throttle opening tonight, as well as the engine earth- just in case I missed it.

    Thanks bro, I'll update tonight :)
  5. Blazer

    Blazer Member

    After reading through this again it sounds like it may have nothing to do with the engine and could be the side stand cut off switch, either a loose connection or the plunger isn't springing all the way out when the side stand is up.
  6. Blazer

    Blazer Member

    One last check would be the wiring at the bottom of the ignition barrel. There seem to be quite a few wr 125's that suffer from wires breaking loose. Not sure if the yzfr 125 could also suffer from this though.

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