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RS 125 Top End

Discussion in 'Modifications and Projects' started by aaronbanana, Oct 18, 2010.

  1. Hi Folks,

    I am in the process of doing a top end on my 2003 125. Considering my experience with this sort of thing its been going well. Hit a snag though once I got the whole thing back together and tried to start it. Nothing happened. I shouldn't say nothing, I had the fuel tank up and I was doing a test start so I could see into the engine. There seemed to be a large spark come out of each battery connection. Not sure what this means. There was no turn over of the engine and not much else happened. Lights, indicators etc work fine. Fuses are fine also. Subsequently have not got spark and have been scratching my head and going through the possibilities. New spark plug still nothing.

    I may have narrowed down to the HT lead (picture attached). I get no reading from the lead that connects to the spark plug into the earthed connection (nor any other for that matter).

    There was no electrical problem prior to work commencing so not sure how/why something went wrong. I am thinking I may have bent the lead back that my have dislodged/broke a connection? Does this sound feasible.

    Any advice appreciated.


  2. Hmmm,

    damn, I spoke to soon. I had the damn multimeter set incorrectly! Am getting good readings for the primary and secondary windings. That leaves me back to scratching my head :(.

  3. Hello again,

    Cleaned everything up, disconnected everything from the plug back to the icu then put it all back to together. The engine now turns over, but I don't have spark.

    Any ideas that would help me track it down?

  4. #4 aaronbanana, Oct 19, 2010
    Last edited by a moderator: Jul 13, 2015

    I have recorded a video of the bike (not) starting. Any advice on that noise? Still no spark, working on it, but thought this might give the more experienced mechanics a bit more insight.


  5. Sounds like its turning over but only very slowly. I would say you have a flat battery.

    Charge the battery fully and try again, or get a jump-start to get it going.

    How do you plan on running it in?

    Cheers - boingk
  6. Cheers boingk,

    I figured that out today after posting. Stupid! Have done so much work to it the obvious problem completely eluded me and was looking for every obscure issue it could be. Fair to say i am not a mechanical person doing the top end onthis has been a real experience i went and ordered a new battery as the current is looking very crusty, worth it for 50 bucks.

    Running in wise was going to follow the manual. Can't recall exactly what it says, under 6000 for a few hundred kays then increase after that.

  7. Righteo, glad to hear you figured it out yourself. Basic stuff gets even the best of us, and its good to work through until you find a solution yourself sometimes.

    I would not follow the manual. For something that only has 10,000km between topend rebuilds, it seems an awful waste to spend 1,000km running in. When I rebuilt my Aprilia I ran it in by using the Yamaha kart engine instructions; Warm up for a few minutes at idle, then spend 10min at 1/4 throttle, 10min at half throttle, and your final period of ten minutes incorporating full-throttle acceleration. It is also important to incorporate a few decelerations from high rpms, but as this is a two-stroke I would not do this a lot as it can cause seizure and/or bore damage if you do it too much.

    Basically, ride to the servo using no more than 1/4 throttle and fill up with 98 octane goodness. Go to a section of twisties using 1/2 throttle. Go through the twisties using full throttle when you feel like it. Once you get back home, check the spark plug and coolant level, as well as other maintenance items like chain tension and lubrication. I would also clean and lubricate the powervalve if you have not done so already - perhaps this is something you could do before the ride.

    Cheers - boingk
  8. Cheers Boingk,

    gave the PV a good clean, what is the lubricate?

    Ok, so the continuation of my story. Got a charged battery. Getting spark, but still no ignition! Have started the bike with the spark plug out and yes, there is a spark there, and pressure coming out of the cylinder, but it does not smell very petroly? Seems like it should to me? After a few turns of the air pushing onto my hands I could not really smell petrol at all?

    Man, this engine stuff is hard. Got fuel in the tank, plugged into the carb. I disconnected the fuel line to the carb and that was full of fuel so it going in. Does it take time for the fuel to make it all the way through? Have pumped the throttle a fair bit.

    Any thoughts?

  9. Where are you at
    Sounds like your not getting fuel
    Your carb could be dirty
    Have you played with the air screw
    Your plugs clean?
    Your breather lines arent blocked or anything
  10. Check you earth connection to the battery...then drop the bowl on your carby to see if you have fuel...there should be a little screw on the bottom you can undo....take off the airfilter and choke the carb with your hand when you wind it over ....you should end up with a wet palm of fuel.
  11. Righteo. Choke on, fuel tap on (or reserve if needed), ignition switch on. Don't touch throttle (or usual procedure).

    If still not starting then take out sparkplug and tip a quarter- to half-teaspoon of petrol into the bore. Try starting again and get back to us.

    By the way, lubricating the powervalve is only supposed to be a light smear of oil (80~90 weight gearbox or regular four-stroke engine oil...not 2T oil). The main part is ensuring it is clean so it won't stick in either the open or closed position.

    Cheers - boingk
  12. #12 aaronbanana, Oct 25, 2010
    Last edited by a moderator: Jul 13, 2015
    Hi All,

    thanks for the help and tips. They certainly worked, here is my first (ish) start.


    I drained the fuel and let some run through the carb. It seemed to work. Fresh fuel, fresh battery, fresh charge all seemed to help. Very happy, taking it for a run shortly.

    Thanks for the help along the way. Couldn't have done it without some of the hints and tips, not only in this thread but in previous ones. Any feedback you give is likely to help others as well as the direct poster.

    My next challenge will be the bottom end, but I will enjoy riding for while before I embark on that one.

  13. Good stuff mate, glad to hear the bike is going. Also very good to hear it on that video, lots of memories of mine - it was my first bike and I built it from a wreck, rode it the better part of 10,000km before rebuilding the topend and selling.

    The bottom end is a bit trickier - should've done it while you had the top off! Anyway, they only need to be done every 25,000~30,000km so I guess its not such a big deal.

    Cheers - boingk