1989 dt125r wont idle after rebuild. HELP!



  • Hello im having a major problem with my dtr i got it needing a new top end rebuild and even with the horrible compression it had then it started and ran fine, after i rebuild the top end i noticed it wasnt idling right or it would only idle above 3000 rpm and when i try to turn the AF screw down its as if it does nothing, i replaced my left crank seal to make sure it wasnt that beofore i split any engine, and that helped a little it took the revs down a bit but still wont idle at 1500 it just dies or if i try give it throttle to bring it up it boggs out and dies, its been doing my nut in for past month or so and im clueless what it could be as everything got cleaned out properly and still doesnt want to idle. Any help would be much appreciated



  • Hi there,

    Always a good idea to introduce yourself before posting technical questions, just out of common courtesy.

    If you are really having troubles then it's best to take it to a professional to have it checked out.

    I'd go out on a limb and say the engine needs an overhaul.

    But it could be a variety things.

    If you bought the bike needing a top end, then that's telling me the bike had some sort of failure. If that's the case did you ensure the rest of the engine was in tolerance before slapping the engine back together. I don't want to be suggesting things for it to be down to a rough motor.

    It foes sound like to me an air leak, this can occur in a variety of places, but most commonly are the left hand crank seal and inlet tract.

    Spraying WD40 in these areas will cause the idle to drop back down to normal which will help identify where the issue lies.

    Other items could be you using a cheap aftermarket head gasket and it's blown already causing water to get into the combustion track.

    Again a tired motor would cause these sorts of issues.



  • Please check your rev cable this seems to be a common problem even if it looks good make sure that it's not pulling and check the adjustments I had the same problem and clean the carb if you haven't already



  • @Calum hello my bad i was just needing some help as i dont normally post on forms, everythig for the top end is genuine and proper sizes i bought a micrometre to double check everything was sound and right, and ive been thinking about sending it to someone with more whit around them i just dont want to be ripped through the arse in the process because i does run just doesnt idle thats all anything below 3-4000 rpms the bike seems boggy, ive spoke had lots of advise from people on what it could be and it is slowly getting better



  • You can't measure the bore with verniers? Not sure how you would measure the bore with a micrometer.

    The bore IS NOT ROUND it is OVAL.

    Therefore you will get different readings depending on where you measure the bore.

    PLUS the bore can go out of shape from excessive use!

    SO it's best to have the bore checked professionally for the size, unless you have the original piston hat you know worked so you can see what size it is.

    There really isn't much to these engines.

    So if it's not running right then it's going to be.

    • Poorly engine
    • Not Jetted right
    • Faulty ignition.

    If you have a sound engine, working ignition and jetted back to OEM standards then it'll run.

    I'd suggest getting a Haynes manual, as this will tell you how to get it back to standard, which is where you should always start when diagnosing problems.



  • The bike has been re wired properly following the hanes mauel im not new to bikes im just knew to this age of bike i think it will be my jets because who i bought it off of had everything cocked about with and all im tring to do is set it standered not make it a beast or a weapon jusy so everything is as it should. And the piston what was in it wasnt the best scratched to death along with the barrle so i changed it all to standered. The micrometre was for the piston not the barrle as its easy to messsure a piston to a barrle im well aware, ill be getting it sent off to someone who can set up my carb because that to me is the problem, when i started cleaning it all the af screw was jammed in and the idle screw all the way in inicating it ran like shit before for best words



  • @Yammy98 Just set the carb to the stock settigns for the year mate. Nothing fancy about it.

    If the barrel was scored then it'll need to be rebored, or at the very best honed. An oversized piston would be needed if that was the case.



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  • Im pretty sure the bore isnt oval, it is hovewer tapered slightly towards the bottom...

    The manual should tell you where to meassure.
    A crude way is to insert a feelergauge between the piston and cylinderm this will give you a ballpark reading..



  • @Jens-Eskildsen Not oval oval, but they are not round. The piston can go in both ways, but should ONLY be put in the correct way.



  • @Calum so let's say you put it in the wrong way will you notice if you ran it like that and do you use those little wire rings that come with the pistons?



  • @declan The engine will seize if you put the piston in the wrong way.



  • How are they supposed to bore and hone a cylinder, and not have it just about perfectly round? It is round to within very fine tolerences, which I wont even bother to look up in the servicemanual, but a guess is a couple of thousands of a milimeter. If you call this conical, fine be me. But then anything is conical.

    The piston has different thickness, not the cylinder. Piston is smaller at the top, because it expands more there because of the heat, so its made a bit conical



  • @Jens-Eskildsen My bad, I was wrong about that sorry.

    It's because the piston pegs foul the ports if it's inserted the wrong way round, not oval.

    Still though, I am pretty sure you shouldn't be measuring the bore wither verniers etc.



  • Agreed on the verniers you cant meassure the cylinder precisely with those.