HiIDT 125 lc 1987 bogging and afterfiring

  • 4 months ago me and my dad bought a 1987 DT 125 lc ypvs, really nice condition, well looked after and very original, we cleaned it up got it running and it was all good, since then it's been sitting in my shed waiting for my 17th birthday, I'd taken it up the road before but it had never really been properly ridden since we owned it, the bike runs perfectly when it's been sitting for a while and it's cold but it seems that after about 2km of riding it bogs down, afterfires and dies and we can never seem to get it running properly (I have a video of it doing this and of it running how it should) we've cleaned the carb countless amounts of times changed the plug, swapped the coil there's defiantly fuel getting to the carb, we have a receipt for an engine rebuild, before we bought it, by a respected bike mechanic in my area from a couple years ago and the bikes not done many miles since then so I wouldn't like to think it's anything internal such as the rings or seals, when the bike runs as it should it feels amazing, its got good acceleration, it starts 1st kick and it's really smooth, the only thing I can think of is the cdi but I don't know much about what they do and how to test them, its got a strong blue spark that has a solid tick sound when it does.

    This is when it boggs and after fires
    this is when it runs

  • Administrator

    @jordang17 Use YouTube to upload a video then post it back here.

    Cracking bike for a 17 year old bud. But please please please, LOCK IT UP! It will get stolen if you don't lock it up I promise you.

    Are you sure it's not just bad fuel.

    If the carbs clean, the engines fresh and there is a good spark, then it's not going to be anything severe.

    I'd imagine it's just draining the carb when opened up properly. Are you sure the carb is setup properly.

  • @calum yeah we have the manual, the carbs all set correctly, we drained the fuel and put fresh fuel in it and I managed to ride it to the petrol station and fill it up so its not bad fuel

  • Administrator

    @jordang17 Yeah, is the float adjusted correctly.

    There's no kink in the fuel line from the tank to the carb.

    Fuel filters not blocked.

    Air filter is nice and clean?

  • @calum definitely no kink in fuel line, we've defiantly messed about with the float to see if that made any difference and I think my dad set it correctly, in the videos we took the filter off cause we were constantly taking the carb off but when I test rode it we put it all back on, we set the plug gap and after I filled it up turned the fuel tap on the on position instead of reserve and that defiantly made it last much longef but it still ends up doing the same thing

  • I've now uploaded the videos on YouTube links in the original post

  • Running bat when hot could be a rich condition, and/or a failing coil, where resistance adds up as it the temperature increases.

    The backfiring has me puzzled, never experienced that on a 2stroke. Have you checked the reeds?

  • @jens-eskildsen I found a video on YouTube of the same bike doing the same problem and in the video he unplugs a green connector coming from the cdi unit and it sorts its self out so it went to my bike it ran like shit unplugged this green connector and it runs fine, I was watching a cdi replacement video from carmo electronics and when he was putting the new cdi in they left the green connector disconnected so I'm not sure what it's meant to do

  • Administrator

    Sounds like you've covered the fueling system, which means it's going to be an electrical component.

    You will want to buy a new spark plug, HT Lead and iginition coil to rule out those items.

    Next will be the pickup, if that starts to deteriorate then it will start to missfire.

    Then unfortunately the CDI. But CDI's rarely fail, but I guess it is 30 years old.


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