About the rubber intake manifold.

[Hark_Ptooie]

I recently derestricted my 2003 DE03 DTR, and it amounted to pulling out and isolating the reed switch in the speedometer with a fat wad of electrical tape, then swapping the exhaust resonator for a tuning part.

Among other tricks I have read is to cut off the rubber lips on the intake manifold. Or whatever that gizmo is named, the one that you plug the carburetor into.

So I bought a brand new one from eBay, because I figured I would experiment on one and have a spare if I b0rked something.

However - when I received it and had a good look, it did not strike me as that anything about it would restrict gas flow. It is a circular passage that bends and transforms into a square "beak" that enters the reed valve cage snugly. The cross-section area does not have any constrictions to speak of.

So I did what any normal person would of course do and spent a few evenings modeling it in 3D and running CFD analyses on it, reed cage and all. Testing various modifications.

And lo: I could not improve it. Removing the rubber lips, rounding off edges, carving away material this place and that - all resulted in something like a 3% flow loss.

The only positive results I got was from slighly rounding out the insides of the square front of the passage. That gained me something like 0.2% mass flow. You would not even be able to measure it on a dyno.

So leave it as it is. It is not a restriction - on the contrary it improves the flow into the reed cage by removing turbulence right before it.

I'm kind of impressed how right they got it back in the 80's, with no CFD software to speak of.

[Calum]

Yeah, not sure why people says its a restriction.

That said, it's not found on YZs, my Aprilias or my Cagiva. All of which are vastly superior.

Furthermore I removed it on mine as I fitted a VForce reed block and that requires it to be modified.

[SpookDog]

Isn’t the reason it’s there, along with the two ‘boost’ bottles and air box design, to iron out any flat spots and optimise lower range torque? And it works!
First unrestricted bike I ever rode was an RD125 that didn’t move unless you were slipping the clutch above 6K no low or mid range power at all.
Tuned 2T’d aren’t for everyone’s ‘taste’ on the road. On the track would be a different matter …

Edited for punctuation to make it ‘more readable’ (cause I was wasted, again!)…

[Calum]

@SpookDog I don't know about that...Cagiva was bone stock when I got it and it absolutely shifts!

Ultimately, any 125 in 6th gear at 4k RPM is going to struggle to pull, but that's okay, just stick it in third

[SpookDog]

@Calum

What kind of masochist would you try and pull from 4K in top gear anyway? (Apart from a 4/ owner!) …

[Calum]

@SpookDog Me on my 170 No maybe not quite that low, but that's what I've come to appreciate with the Yamaha now.

You look back 15 years ago when I first used the Athena kit and you'll see a completely different opinion from me.

But now I have my other screaming two strokes, I am happy to have a DT which doesn't need to be revved to the moon to enjoy. But oh boy I've had some good rips home from work this year on the DT