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Bike wont Rev can someone help?

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  • Bike wont Rev can someone help?

    I have this old 82/3 ish XS250R Yamaha which just will not rev. It is not the cruiser type XS250, its the one with the big square headlight and looks like a commuter/sports tourer bike from the 70's It is now surplus to requirements as I have bought another bike to do the job this one was going to do and I want to move it on but it has to be right before I sell it.

    It has a weird weird problem.

    Sometimes you ride it and its all good, most of the time it gets to between 5 and 6000 rpm and its like someone just turns the lights out and it will not go past about 6k, however put your foot on the brake pedal or pull the brake lever in and it will hunt as if it wants to go but only for a split second as soon as contact is made with the brake circuit.

    It has new plugs and the carbies have also been cleaned out.
    I have put new earths to the engine, frame and one directly to the CDI from the battery which has had no effect.
    The CDI internals look pretty good with bugger all signs of water damage or corrosion and the coils look new(but are probably 30 years old)

    Only fault I have managed to find is that one of the plug leads was a bit dodgy but I think I have fixed it. I reckon this would just make it not run rather than stop revving over 5kish.

    Any help will be appreciated

  • #2
    I'm betting on an electrical breakdown at higher revs.
    Sure that plug lead is fixed?


    • #3
      Dont forget a coil that is fine when the bike is cold can breakdown when hot.
      You put the c*nt in country run


      • #4
        Does it have points ?? Worn backing plate, wrong gap ?? If iots the model I think then maybe not but worth a look.



        • #5
          Mate unless you have a broken plug lead fixed professionally you can almost never fix them yourself....they might be ok at low revs because the energy output is not too much that the breakdown in the lead leaks.

          But at high revs the breakdown becomes apparent..... an easy way on a car is to wait till dark.....turn all the lights off and start it..... and go through the rev range and see if you can see any spark jumping from the brokendown lead to the engine.....

          Hope you get it sorted


          • #6
            Cheers for the input guys but I seriously doubt it is the plug lead. I have ordered a new set which will get here eventually.

            It has to have something to do with the fact when the brake lever circuit is activated it does something to the ignition circuit as this is when it picks up and runs properly.

            Coil temperate could be a factor but it plays up most of the time. I might see if I can borrow a set of coils and see if that helps.


            • #7
              A wiring diagram is definatly the go...I know from memory some ignition circuits are wired via the brake light circuits...for some reason I could never figure out. I would be checking the wiring to and from the brake switches and the lighting circuit.

              Good luck
              Smoke me a kipper...I'll be home in time for breakfast


              • #8
                Volt drop in the ignition ciruit due to brake light allowing the points and or switching transitor to for a brief moment to switch better due to reduced load.