Problems with my Poorly maintained 2002 DT only 6500 miles



  • Hi calum I am going to get a rebore and new piston etc for my DT 125 R ,which piston set is a good buy and which ones should I steer clear off

    3 days ago Calum
    You can't really go wrong with any of them. Throw a few names at me and I'll let you know

    I personally always buy Wossner forged pistons.

    But I'm running more than stock power

    I have racked up 10K plus on cast mitaka pistons

    And I've racked up 1K miles on forged. Can't say I noticed a difference

    Only be careful with Wossner as the piston ring end gaps are always too smal

    2 days ago Leftyno1
    Thinking of going with the Mitaki piston, bike only had 6500 miles when I got it so disappointed to be working on it, interesting point that Piston was only worn on Exhaust port side , piston scored entire length and rings stuck in grooves on same side very strange.

    a day ago Calum
    Not really sounds like what you would expect. The exhaust port will run a lot hotter than the intake ports. Sounds like it over heated to me. These bikes will do around 10K to the piston. More if you use decent oils and look after it. But sounds like heat seizure to me which would say you might want to look into the fueling/cooling. The colder weather will make a two.stroke run lean. So you need to up the fueling.slightly.

    a day ago Leftyno1
    my bike is running in a constant 80 deg heat ( Caribbean ) so that could have been a factor, I took off the radiator cover to allow more air pass through, plus power valve was not working it was in closed position, could that have been part of overheating problem as under full throttle exhaust gasses can not get away ??

    a day ago Calum
    Put the scoop on mate. Removing the scoop causes negative air pressure behind the radiator which in term means less flow and less cooling. 80 degress isn't too hot which could tell me its either a fueling thing.

    a day ago Calum
    Curius about the power valve. Are you sure the powervalve didn't hit the piston?

    What bore size you on. If its an oversize then you need to modify the powervalve.

    about 11 hours ago Leftyno1
    Hi Calum it is standard bore original Yamaha piston which is 56mm ( think lol ) powervalve when rotated is not near bore as I thought it could be that maybe, running a 220 jet in a brand new VM28 Mikuni flatside carb,needle valve 1 notch lower snorkel out air bottle out on boot ( hole taped and sealed),reed switch removed from clock and placed out of clock, ,manifold above reeds sq rubber nose removed and filed smooth, exhaust is stock with a KH125 tailpipe ( on when bought ) and have only done 50 miles on it since bought [there was no air filter in when bought though and had not been run for a year, changed coolant also, thats it as they say.

    about 11 hours ago Leftyno1
    I always thought the bike might be low on compression as I did not feel a lot of resistance at TDC, but I thought hell its only got 6500 miles so must be my imagination,oh exhaust was loose at manifold when i bought it oil all over exhaust

    about an hour ago Calum
    Oh dear

    Those bottles want replacing and putting back in

    Put the main jet to 240

    about an hour ago Calum
    Put the snorkel back in. There is nothing to ne gained removing it

    Starting to sound like incorrect air/fuel ratio caused it to run lean mate

    Amazed it ran for as long as it did if thats the case. Take the spark plug out and have a look at the colour

    And this is starting to turn into a thread. Do you mind posting this in the correct section

    This will not only allow others to chime in

    But it was also record this information so that others can read it shoukd they experience something similar

    2 minutes ago Leftyno1
    No problems, will do cheers



  • Does the barrel need a rebore. Or can you get away with a hone. Throw some.pictures up so we can see.



  • OK will do when you tell me how or where to upload pictures LOL.



  • You will need to upload the image to a hosting site, I use photobucket. It's free.

    Then you need the image URL. Simply post that into the text area and it will appear.



  • Great will do that in morning.



  • 0_1491029202592_Barrel_zpshrk8dtqd.jpg



  • 0_1491029257821_piston 2_zpsju3plqan.jpg



  • That head will want a gentle skim. I wrote a tutorial about how to get that done.

    Get the barrel down to a specialist to get them to see whats what. Can't see from tbe photo but it might not be too bad.

    I would make sure the oil pump works correctly, increase the oil slightly or bettee still go premiz.



  • Could be wrong but almost looks like death ash on the piston crown. Another sign of overheating.



  • Thanks for advice bringing all the bits back to UK at Xmas ,to a good Mechanic in my area for re bore etc.
    The pre-mix idea is a very good one as I read on a blog about a guy who had 3 engine seizures before realizing it was the oil pump on his DT, In my humble opinion I think the previous owner ran the bike with no airfilter loose exhaust manifold which would in itself cause overheating plus PV not working and in shut position,
    So on the bright side I think the CDI is OK as spark plug sparks 😆



  • Okay,

    Firstly CDI is not the only thing you can determine from the plug colour. I was thinking more fuel ratio since cdi does not control fueling. And cdi tends to be stock ignition curve.

    Yes air filter will cause premature wear of the piston. But this would be evident in the reed cage and piston walls.

    If it were me I would restore all the missing bits. Boost bottles snorkels and up jet it to 240 main jet.

    I would replace the exhaust stud. I can't see why this would cause it to overheat though. I appreciate the scavenger process will suffer because of the harmonic resonance charge lost because of this stud. But then power would be lost too so it would only run a little hotter.

    But whatever.

    Keep us posted



  • Will do Calum ,all good advice so will put everything back on and jet up, will report on progress after rebuild. Thanks.



  • Sounds like someone read on Facebook that they were all restrictions so removed them. Forums are the best for this sort of information and you have come to the right place.

    The gain/loss might not be noticeable. Certainly the sound will bw different. But trust me when I say it's for the best that it be restored.



  • Oh just as i am curious what does that bottle do before the Carb inlet ? , and is it worth taking baffle plate out of Exhaust ( mine is in the middle section of exhaust have to cut open and weld up), cheers



  • They are boost bottles. I can't remember the physics. But something to do with resonance.



  • I took the baffles out my exhaust before. Not worth the effort if you ask me. Just go ahead and buy a system.



  • Cause there is nothing to be gained removing the snorkel, wat a load of bs. Put the right pilot jet in and set the mixture screw up right and there is loads of bottom to mid to be gained.