OKO carb for ETZ251

[Puffs]

Just to keep you posted, I have gone back to the Koso.

The reason was that the BVF seemed to develop some sort of blockage: it gave the impression I was running out of fuel, and after that it ran fine again. So I took it off, and put the Koso back. With that it runs fine, just as it did at first with the BVF, when I put that on a few days back. Strange. Flow from the fuel line is fine, could it be something like a sticky valve in the carb? I'll have a look if I can find something.

edit: and I did, see viewtopic.php?f=5&t=10596&sid=25435ba10f77e847187faa873e2d6543#p51635

[Puffs]

One thing that was still outstanding is trying how the Koso performed without that Power Jet. The modest engine power allows looking at the pipe connecting the Power Jet to the float bowl, also at full throttle, and with the fuel coloured red by the premix oil you can see the PJ aspiring fuel. At 2/3 throttle or less, the pipe remains empty, and only at (or very near) full throttle the PJ's aperture experiences the venturi effect and aspires fuel: that's when you see the level slowly rise & the pipe fill up. It is then when the Power Jet starts to make the mixture richer, but if that is required is another matter: most carbs do not have it, neither does the original Keihin PWK, nor the MZ's BVF 30N3-1.

Now you can block that Power Jet circuit by installing a blank jet (which I do not have), or by removing that jet & using a much taller cover screw (which may leak a bit), or just by putting a plug in another piece of pipe, as shown here.

Koso PWK32 with Power Jet blocked.jpg (23.6 KiB) Viewed 45 times

That also makes swapping with/without easy.

Back on the road, the difference is very small. On my test stretch it might be 1-2 MPH faster without the Power Jet, and in the lower gears it seems to pull a bit harder at high revs.

After shutting down directly after a 2km full throttle section, the plug looks as follows:

Plug after long full throttle, no PJ.jpg (37.5 KiB) Viewed 45 times

It looks hardly different than before, and that is in line with expectations; the power jet is pretty small (0.35 < PJ < 0.85 mm), and its aperture is flush with the top the carburettor's bore.

Anyway, I'll keep that Power Jet blanked off for now; the conventional 1.40mm main jet will suffice.

[Blurredman]

And yet I thought a plug that looked like that was too lean?

[Puffs]

Nah, I don't think so. See Haynes pg. 27:

Haynes ETZ, P.27.jpg (39.43 KiB) Viewed 45 times

Btw, it looks like Haynes & NGK share these pics://ngksparkplugs.com/en/resources/read-spark-plug
For me, particularly their 'Normal-Condition.jpg' looks comparable:

Normal-Condition.jpg (8.68 KiB) Viewed 45 times

That's even whiter, but that might be because of some colour editing.

[Puffs]

Not being entirely satisfied with this, I'm also adding some pages on tuning from the YZ250 Owner's workshop manual. This pertains explicitly to a 2T engine (with a recommended premix of 1:20/1:24), albeit water-cooled and with a reed valve on the intake & a power valve on the exhaust.

Plug reading from YZ250 Owner's workshop manual.zip

[alexxx]

Great work Puffs,
Your koso carb is the same as the OKO I bought, they are all Kehin clones.
I bought the made in Taiwan model as they are meant to be the best of the clones. (http://www.mid-atlantictrials.com/Sales.html)
Good web site with lots of helpful set up info.
I fitted two of those OKO carbs on a T140 I have and they are fantastic, easy to tune and very cheap for the hex jets, also a nice light throttle, the small size helps a lot.
I expect the same results as you when I fit the OKO but the ability to be able to tune the carb with the huge variety of jets could make it a better carb to live with.
My OKO is a 30mm, I chose that as I am not intetested in a higher top speed and lesser dia could help the bottom end a bit more..I think.
I will have to buy another inlet manifold to modify.
Regards

[Puffs]

Ta. In a way I regret having bought the 32mm version; not that it performs poorer, but the extra 2mm D is just not required, if you leave anything else standard. Well, the 32mm might perform poorer on low revs/full throttle, so best avoid that.

I liked the solidly-built chromed slide, the easy access to the main jet,

Access to main jet.jpg (17.83 KiB) Viewed 45 times

but in my view that 'power jet' is quite pointless. It gets it fuel supply via an external transparent pipe (from the bowl, obviously), and when you open the throttle fully that power jet gets exposed to the venturi effect and starts to draw fuel. And indeed, if you look at it when keeping the throttle pinned, you can see the level rise, and reach the top after a few seconds. But it does take a few seconds, and that seems wrong to me. If it needs the extra fuel/richer mixture, it would need that instantaneously (and the main jet reacts instantaneously). What's the point of the mixture becoming richer after, say, 5 seconds? So I just blocked it off & adjusted the main.

Koso with powerjet blocked.jpg (19.97 KiB) Viewed 45 times

Now it is of course possible that my 32mm the velocities are smaller, so that the venturi effect draws a smaller vacuum. The 32mm has a 14% bigger x-sectional area, so that also the velocities are 14% lower, which might make jetting more difficult. Anyway, for me it was just a try, and I went back to the original carb. But I can see how such a carb would do well on a Triumph!

[Kruh]

When I had the 30 mm OKO version, i too found the power jet not making a whole lot of difference.
I think MZ are just too low revving to take advantage of that. In fact I almost never find myself going over 3/4 throttle, even at harder accelerations.
I've also seen people installing 28 mm carbs on 250 MZs. And they seem to be plenty. While offering better low end performance due to the intake resonace shifting lower in the revs. I assume the increased velocity might help tuning as well.
Although, I'm very happy with my current version of the 30 mm (ZSDTRP).