DT 125 R '88... New project

[jgr72]

@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

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 WR200 should have more horisontal angle to avoid rear shock and that joint is originally made for TMX30 carb.

[Calum]

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

[jgr72]

WR200 inlet did the trick I need:

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!?

[Calum]

@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.

[jgr72]

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...

[Calum]

@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.

[jgr72]

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!

[Calum]

@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.

[Irongamer727]

@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.

[Calum]

@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.

[declan]

@calum are there any of the aprilia engines that don’t have right hand drive?

[Calum]

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

[declan]

@calum well uno out of the Rotax engines but what if it’s an sm

[Calum]

@declan Just get an mx mate, much better bikes.

[declan]

@calum ahh okay

[jgr72]

Bike is getting ready for next summer:

That exhaust chamber :smiling_face_with_heart-eyes:

Gianelli silencer:

But need to twist something:

I guess it's easier to warm and twist + cat a little bit of that exhaust chamber tail pipe... Gianelli's aluminium would melt away too easily...

[Calum]

Looking lovely, I'd be reluctant to bend the pipe, just make up a larger bracket and move it closer.