DT 125 R '88... New project



  • I must admit, this is probably one of my favourite colours for these bikes.



  • @jgr72 nice๐Ÿ˜‰



  • Few days ago, I took the engine in parts... and this is what I found:

    alt text

    Thankfully new parts arrived already:

    alt text

    I tried athena barrel and piston, and it seems that tdc is just as with the earlier barrel & piston.

    I will measure both heads and calculate compression ratios just for curiosity...

    Still waiting for piston pin bearing to arrive... Old one was just fine, but why not change it to a new one, when it can be done easily...



  • Doesn't look too bad, barrel should hone nicely, might not need to touch the bottom end.



  • Yeah barrel might still be useful... I will store it carefully, so I could return this bike original later if I want...



  • @Calum Athena 170cc set fits well into this 3BN engine, there was no problem to install it. Original piston remain about 1-2mm below deck and dome comes just about up to deck hight. Athena piston comes to deck hight and the dome is smaller and comes just over the deck. Athena claims that compression ratio is 13.2 and that is also what I endup by calculating it... combustion chamber is about 13ml and 168cc devided by that is around 13. That's not actually the real CR, since 2T engines start to compress only after the channels in the barrel when piston comes up, damn I didn't measure that part of piston travel...

    Next step is to find better solution for TMX30... I ordered from ebay WR200 intake, let's see if it will line the carb better past the rear shock. If not, I may need to build up my own intake... At this point it seems that the DT230 Lanza boot was a waste of money, it does'nt seem to fit in any way... Anyone interested?

    I also ordered 170cc version exhaust from Romeu Henriques, let's see how it fits and feels... Silencer will be a gianelli alu silencer...

    Anyone have idea, where to start settings of that TMX30? 135 mainjet felt best last autumn... Maybe 140-150 mainjet and needle one step up?

    And for all that are oppressing me to keep this as original as possible, that's not happening now... I will store all original parts for later time if I feel to return this original back again...



  • Sounds like you caught the tuning bug, very serious this is.

    I am surprised you needed a bigger inlet for such a small carb. You can get reducers to fit.



  • @calum said in DT 125 R '88... New project:

    Sounds like you caught the tuning bug, very serious this is.

    I've had it since 1987 ๐Ÿ˜‰

    I am surprised you needed a bigger inlet for such a small carb. You can get reducers to fit.

    TMX30 will fit quite well to original joint, but the angle is bad... WR200 should have more horisontal angle to avoid rear shock and that joint is originally made for TMX30 carb.



  • @jgr72 So I'm running a Dellorto VHSA 32mm on stock inlet manifold with no issues.



  • WR200 inlet did the trick I need:

    alt text

    Air filter barely misses the rear shock and frame...

    Now need to modify those inlet attachment bolt holes to make it fit into crankcase...

    Maybe I will loose some bottom torque due to the missing boost bottle!?



  • @jgr72 Personally I'd want to keep the boost bottle. Whether it actually does anything though as you say.

    They did have boost bottles on some WR models. I think it's more useful on road going bikes.



  • Yeah, I'd like to have it too... It have been there for a reason... Okey, maybe I need to try to find such a WR inlet that have boost bottle! This bike will be used 99% on sand or asfalt roads...



  • @jgr72 I'm sure it'll be fine, you went with the big bore kit and the way those boost bottles work is via harmonics. So it's doubtful the frequencies would have been correct anyway.



  • Hmm... that's true also... So if I want to use boost bottle, I need to enlarge it about 35% to be in the same as previously 125cc!



  • @jgr72 I think it's a little more complicated than that. As said it's all to do with Harmonics and the resonance frequency of the engine at a particular motion in the engines cycle.

    I don't know the specifics in intimate details but the idea is this.

    At some point in the engines RPM cycle, there is a dip in the air pressure and consequently the engine will perform less than optimally. They characterised this phenomena with a certain frequency. They then created a body with a natural frequency that matches the signature of this phenomena. When the engine hits this frequency, and is down on oxygen, the bottle resonates, forcing reserved air into the inlet tract. This makes up for the loss charge during this period and as a result performance is restored.

    Now to my mind, there are so many factors that have to be considered that it's either pure genius or pure myth...it's hard to say whether the performance with or without is noticeable, it's probably so slight but the Japs just over engineer it all.



  • @calum was it really worth changing to a 32mm carb? I mean it's only a 4 mm difference from stock? Thought those 28mm mikuni carbs were well up for the job. I'm still running the 28mm with the 170 kit.



  • @irongamer727 It's not just the fueling, it's the amount of air you can get in as well. My brother ran a 38mm Keihin carb on his, although to be fair that was probablyt way too big. 32-34 is about right I'd say. The Aprilia RS125 ran 34 as standard and they were making 26-33bhp.



  • @calum are there any of the aprilia engines that donโ€™t have right hand drive?



  • @declan Aprilia didn't make their engines. Just swap the swingarm when you do the engine. Simples.



  • @calum well uno out of the Rotax engines but what if itโ€™s an sm


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