Reading the Spark Plug Color



  • Let me start with acknowledging how organized and helpful this forum is , in my attempts to repair and built my Dt the last months. And now back to the point 🙂 !

    I noticed that when I was on WOT the engine was bogging down, and after 100Km of riding, my squeaky clean exhaust was dripping a black smoo thing .. 😑 After I learned that this is a rich condition caused by the main jet, I found out that the main jet was corroded ( or bronze sickness infected ) with no visible markings and when I went to a store with it so I can buy a smaller one, the seller there found the size to be close to 260-300 😮

    Anyway, I started with a 150, now using a 190 and I’m trying to determine if the color of the spark plug is correct. I follow the method described in the Carb Tuning, after cruising with 3-4th speed and mid rpms for about 15 mins, I go WOT for about 10-15seconds, I hit the kill switch , stop and examine the spark plug. These are the results :

    Here you see the "original" compared with a 150 i got for testing 0_1537187045130_Old_main_jet.JPG

    Doing the spark plug test with the 150 i got this :
    0_1537187365958_Spark_150.JPG

    Doing the test with the 190 :
    0_1537187710921_Spark_190.JPG

    After reading a lot of guides I still can’t tell if I’m closer to lean / rich or ok.. Any insight would be much appreciated !



  • @panmil92 looks pretty much spot on maybe ever so slightly rich but I’d run that maybe someone else has better info



  • What is the spec of your DT?

    Mikuni TM28 flatslide carbs should run a 210 or 240 main jet. The black good dripping from your exhaust is more likely unburnt two stroke oil, and that plug seems to indicate actually you're flooding the engine with 2 stroke. Rather than it being rich or lean.

    Unless your engine is modified, I'd suggest return it back to bone stock. Adjusting the oil pump back to factory settings, restore the air snorkel in the air box and go from there.

    When tuning, start from a base and only adjust one part at a time. That way you can understand what affect the change is having.

    A perfect colour is "Golden Brown". It is very distinct and you will know instantly if it's right from the colour.

    But that only checks WOT fueling, not under load up a hill at part throttle. Which is why bone stock DTR's are very good, since Yamaha have worked all this out!

    alt text



  • @Calum I followed your advice with the husky it wouldn’t fully rev out so I took the main back to stock and it’s perfect now it was 20 to big stock is a 400



  • First thanks for your replies ! Now it’s a DT 125 R 1997 , with mikuni flatside TM28SS. I bought it second hand , the piston was breaking apart ( small pieces had broken up and scored the cylinder but it was still running no problem ). Anyway I exchange the cylinder from a second Dt engine I have, the cylinder has 8000 km on it and just to be on the safe side I took it to a shop , they checked it out and gave me a new piston, rings , small bearing and Athena gaskets.

    This is my try to tune the carb , after the top end rebuild ( I also followed the info from this side on how to do a top end rebuild and run-in ). The snorkel is in, but it’s not a tight fit with the air box.. ( I mean it has some play, it’s not like a solid piece of rubber ).

    Regarding the oil pump, I checked it following the Haynes manual an I thinks it’s ok. ( I don’t have the specific tool for this so I measured some pieces of copper foil with the caliper and used that as a reference ).

    The exhaust is stock with no obvious restriction. On more thing to consider regarding the oil is that I still run the one that the bike came with… Unknown age or brand…

    The other setting of the carb are stock as the rest of the engine. It idles stable at 1.5k rpm and the revs drop according to the throttle position so I don’t think It’s leaking air ( at least not in a noticeable way ) . The rest of the engine and the bike has 38700 km on it .



  • @panmil92 I posted a few links on this thread, which may offer some help???



  • @NINJA I lost you there.., i don't see something in this thread ( this is my first thread on this site , so i may miss something ).



  • @panmil92 Did you have the cylinder honed? Its not enough to simply through in a setof rings a d a piston on a used bore. The piston rings need to seal on the cylinder walls.

    Whilst I doubt thats your problem its worth checking.

    Are you getting excessive smoke? That plug looks too oily. I winder if the cranjkshaft seal is starting to weep.



  • @panmil92 WHOOPS!!! My bad I forgot to add the link. 😁 😡 😖 😣 😆 😉

    See here; https://dt125r.co.uk/topic/1590/spark-plug-heat-range-advice



  • @Calum I took the cylinder to a well known shop for doing honing/ reboring , etc ( in Greece ) , he actually checked the cylinder for clearances as well as condition ( for almost 40 mins ) and then he gave me the correct piston. So I guess ( hope ) that honing is not the issue. I wouldn’t say I get excessive smoke. It smokes in the beginning and under hard acceleration , no more than what I see on youtube with this bike..

    About the crank seals I have no idea, I check the oil level when warm and it seems the same. Unfortunately with the university, the only period of the year I can do the bottom end is the next summer. ( I’ll probably do it anyway then cause the old one had a little surface rust on the counterweights and I want to replace the bearing as well , i just hope it doesn't give up in this year 😇 )



  • @NINJA Thanks ! No worries ,I actually visited that thread when I was browsing the site 😛



  • QUESTION : Will a dead battery affect the performance of the bike at WOT ? .

    On my way driving home, I noticed my horn was not working ( when a real gentleman driving an Audi decided to cut in my lane , as I was accelerating, with no flash/warning 😤 ) and after I arrived home I checked the battery to find a dead cell. The max voltage of the battery under 0.5 Amp load was a steady 10,3v. No load voltage of 12.6v.

    In the same day , before I noticed the battery issue, I replaced the main jet to the manual recommended 240 ( from the 190 I was running as a test) and the bike starts to bog the moment you pass the ¾ of the throttle.

    So, will the dead battery make my bike feel over-rich as I go WOT or the two things are not connected ?



  • @panmil92 Yes, if the bike is failing to generate enough power, you won't be able to fire the ignition as many times as necessary. This will result in the engine bogging down due to it misfiring.



  • @Calum Yeah, I was thinking the same… I am attempting to do a single cell charge to try and revive the battery . I’ll look for a new one tomorrow anyways .



  • @panmil92 I'd be investigating the generator if it were me. At the end of the day, the bike should operate without a battery....



  • @Calum Hmmm, I measured the voltages at both the charging and the lights outputs. They are exactly on specs. I have no other idea on how to check if the ignition pulses are correct ..

    If it’s not the battery , then I’m looking again at the rich problem regarding the main jet.



  • @panmil92 Having a faulty battery can cause issues.



  • @Calum It will be my priority tomorrow. I’ll fit a new battery, test the response of the engine under all throttle positions and then if necessary I’ll lean the main jet . I will of course update the thread when I have more information 😉



  • @panmil92 My 2003 dt 125 r runs exactly the same without the battery and nunning without a battery there is no way at all that it can effect the performance of the bike like @Calum said doesn't make any difference with no battery unless faulty electrics or generator etc. Without battery fitted and multimeter on battery wires when i rev the bike a little it says 15.8 volts



  • @2stroketerry I have the same voltage reading without the battery, with battery fitted it drops a tiny bit to 15 V.

    Since the problem only occurs at WOT , I’m leaning towards rich main jet , even though it’s a 240 as stated on manual .

    I managed to charge the defective cell ( kind of, it holds 2.1v with a 2 amps load ), so I will run a test with the battery charged. If it still bogs at WOT, I will fit a 220 jet and re-test.