TS 250 PWK carb

[Kruh]

Been looking into replacing the needle, since it is still a bit rich around and past 1/2 throttle.
There are needle sets available, but I'm debating wheter I should buy just the needle I need or the whole set. Since the price is a bit high for my liking (cost as much as the whole carb).
In any case, first I wanted to identify the needle that comes in these carbs, and after a bit of searching around for the correct chart, I finnaly found all the necessary info.

The needle that comes in these replica carbs is a JHQ.

I attached the chart down bellow.
Hope it helps.

[Puffs]

Thanks for that list. Interesting, but slightly confusing. I suppose it comes from Keihin?

So you're saying, the needle that comes with these replicas is a JHQ, wherein:
J --> taper = 3 degrees
H --> L1 = 24.05mm
Q --> D = 5.05mm

Do I understand that correctly? I'll have a look what mine is.

On your question, if you can't achieve what you want with another combination of needle height & main jet (needle 1 clip higher, jet 1 size smaller; do a plug chop!), maybe a slightly lower tapered needle might help.

I tend to go the other way round: first the full throttle section, and then the rest. Mine might actually be slightly rich too, as I noticed it runs better in the current cold than it does in summer. But of course, better a bit rich than too lean.

[Kruh]

So you're saying, the needle that comes with these replicas is a JHQ, wherein:
J --> taper = 3 degrees
H --> L1 = 24.05mm
Q --> D = 5.05mm

Do I understand that correctly? I'll have a look what mine is.

That is correct.
Except one typo; the diameter is 2.505 mm

I can confirm that all three needles I got with these carbs are exactly the same. So I assume the vast majority of replicas have the same one.


Well, first the main jet, than the rest is the correct way of doing it. But I have done that.
I very happy with the pilot jet and air screw (I thinks its spot on) and I think the main is very close. But the mid range has been a bit of a struggle.

[Kruh]

This chart is a bit easier to understand.
Also these are all the needles that come in those 10 piece sets.

[Puffs]

Yes, sorry for that typo (needles are not 5mm in OD). I'll get back later on what needle I have in my Koso PWK32.

It's easier to assess a needle following that Polini table, as you don't have to calculate the angle of the taper.

[Puffs]

So I had a look. First some comments on these charts:
1) In the Polini chart, the needle diameter ᴓA would be given by the last letter of the code from the Keihin chart. It is rounded down a bit, yet the KLK at the bottom should be 2.45mm
2) The taper calculated from the Polini values agrees roughly with what the first letter in the code suggests, but not precisely; within typically 0.1° (= 6').
3) The Polini needles are quite different from the Keihin ones, but of course so are the carb venturi sizes.

In my KOSO PWK 32 I found a needle without any marking stamped on it. It was slightly longer than what Keihin specifies for the 26-28mm PWK's, in mine the L = 57.9mm. It is clear that when the venturi size increases you need a longer needle. Yet min has a lowish taper, of around 2.63° (2°38'). The OD = ᴓA was pretty much a standard 2.50mm, so it comes closest to a HFQ. B, C & D were respectively 0.965, 33.43 & 18.15mm.

[Kruh]

Looking at your numbers, I noticed I made a few mistakes. First of all, my taper turns out to be 2.6° (accidently rounded it off to 3). Later I also realized, the keihin chart measures L1 from the first clip notch to the start of the taper. Which I didn't measure.

The lenght of my needle is 57.8 mm.
Taper lenght, C = 33.7 mm
Diameter A = 2.50 mm

So I'd say you are correct, its something closest to a HFQ.
To me, they seem to be the same model, accounting for a margin of error. Whether its down to our measurements or the actual manufacturing of the needles. I mean, it is a bit tricky to pinpoint the exact start of the taper.

Thanks a lot for your input. Wouldn't notice my mistakes otherwise.



Also, I mixed some thing up redarding the jetting. It is slightly rich under 1/2 throttle, when on 2nd notch and lean at around 1/2 to 3/4 throttle, when lowered all the way.
So, from the commonly available needles, a JJQ needle might work. Since A is the same, but the taper starts later (should help low throttle), and with a steeper taper, should allow for more fuel in the mids. Maybe even the JJS might work. Others have a smaller A diameter, which wouldn't work.
Will look further into this, there a a few more models available in a few webshops.

[Puffs]

Thanks for your reply.

Yes, it's a bit difficult to determine exactly where the taper starts. I used the micrometer, setting it (fixated) 1 or 2 hundreds below the size of the bit above (2.50mm), and then approach from below, thus feeling the end of the taper. But that's not very accurate either, and you'll always estimate it too deep.

The other thing is: I do not know what the distance from the top of the needle to the base of the 5th groove is, in the needles these charts pertain to. I have estimated it (using the Keihin pic), as 6.3mm, but it's just an estimate.

And the third thing is this. My Koso was delivered with a so called 'power jet'. That's an extra opening at the top of the venturi, upstream of the throttle slide, a bit before it. The idea is (presumably) that particularly at full throttle, the venturi effect sucks a bit more fuel through that channel, thus providing a richer mixture. Why that would be desirable I wouldn't know. There is a flexible & transparent tube going from that 'power jet' aperture, at the top of the venturi, into the float bowl, where it sucks the fuel from. Obviously with the engine off, or running at lower revs, there is insufficient ΔP to raise the level in that transparent tube, and then that extra channel has no effect. But on full throttle, high revs, there is sufficient ΔP, and you can see the fuel rise in that tube. If you look at it, you can see it takes a few seconds for the level to reach the top of that tube, where the relevant jet is, and only then it can start to disperse that extra fuel. So you get a kick of extra fuel, but only after a while in those conditions.

At throttle openings a bit below full, the 'power jet's aperture is shielded by the slide, so that air velocities are lower there and the venturi effect's ΔP is less. So then you don't get any extra fuel. And at lower revs you don't get any either.

While carburettors are always an approximation in getting to an ideal mixture, I don't feel that 'power jet' is at all beneficial, so I blocked it off. But quite possibly they gave the needle in this carb a lower taper to compensate for the 'power jet'? Note the original Keihin PWK has no power jet.

But more importantly, these numbers on the needles represent/suggest an accuracy which is practically impossible to manufacture. Certainly if you take into account what these carbs cost (in the 10-25€ range for these Chinese replicas, substantially more for the original Japanese Keihin).

Actually, my ETZ251 ran fine with this cheap carb. But it also runs fine with the original BVF, so I put that back. For me that Koso was just a try, a folly (yet I regret not having bought the 30mm version). And I haven't gone all that deep into optimising the jetting, nevertheless I wonder: would it be beneficial to fit an oxygen sensor (λ-Sonde) in the exhaust? It would be a bit of modern technology added to an ancient issue, and maybe it'll be over the top in this sort of application range.

[Kruh]

The power jet mainly has an effect on full throttle anyways. Which is an area where MZs rarely are, atleast from my experience.
It is very rare for me to use full throttle, because the bikes revs out way before that.
I did not notice any difference either when I used an OKO with a power jet.

Some people have been using 28 mm carbs (common size). Claiming better low end power. Which makes sense, since a smaller diameter carb would put the intake resonance lower in the rev range, thus increasing power down low. Also, it should make it more responsive to throttle inputs.
Considering all of the above, I do wonder if a 28 mm carb was a better option overall, even for me. Let alone a 32 mm like you had.

Actually, my ETZ251 ran fine with this cheap carb. But it also runs fine with the original BVF, so I put that back.

So you did't find any major differences between the two in performance or drivability? Not sure if you already mentioned it somwhere else.
If so, it is interesting, considering the pwk was oversized an, like you said, not perfectly jetted.
I sadly wasn't able to compare, since my original bvf was very worn out, so it wasn't even worth bothering with.
Although a replica, its still a modern carb design and I would expect it to slightly outperform the bvf, when jettet properly of course.

[Puffs]

On riding at full throttle: I guess it depends on the rider & on where you ride. If it's in a city or built-up area, true, rarely full throttle, and I keep to the indicated speed limit. But on twisty country roads, which is what I do mostly, if I ride alone there is a lot of full throttle on the ETZ, or actually with any bike.

Yes I regret having bought the 32mm version. At the time, that choice was driven by the spigot OD they specified (40mm if I remember), which for that size carb was the same as for the original BVF. I was planning on using the original intake manifold. But when I received the carb, I didn't think that was such a good idea, so I made a dedicated manifold and used the rubber muff (which was included). I should have bought a 30mm version, the size that has been standard for both TS & ETZ 250's.

Yes, common wisdom (for non-reed 2T's) suggests that a smaller carb gives more low-end focus, while with a bigger carb you should get more top-end power. It ran well with the KOSO, but on top speed (on my test section) it didn't make any noticeable difference, compared to the original BVF. I did have the impression that with the KOSO it was slightly stronger in the lows (which was not expected), and that the surging tendency was maybe a bit less, but in all the differences were small. It all got a bit subjective, and after several hundred miles with the KOSO I went back to the BVF. Maybe I'll try the KOSO again, it's an easy swap (I made the manifold such that the lengths match: the flat-slide KOSO is shorter than the round-slide BVF). And jetting-wise, I was quite satisfied with the KOSO PWK 32, the jetting was fine for me. But indeed, maybe I should try a 28mm.

On outperforming with a more modern carb: it depends where the restriction is. If the carb is the restriction, holding performance back, then yes, then you could expect an improvement from another carb. But if, for instance, the exhaust, or the points ignition, is the limiting factor for performance, less so.

Btw, I said before that it might slightly rich on the main jet with the BVF, because I felt it currently, in the cold, runs better & faster. At lower temperatures there is more oxygen per litre of air, so if that makes things better, it means that at higher temperatures it would be a bit rich (I do not adjust the jetting in the MZ for winter/summer temperatures). But I've realised that another reason for this subjective impression may be that I do not ride the Ducati in these winter conditions (salt). I noticed that the subjective impression of a bike is quite dependent on which other bikes you rode recently. So maybe the jetting in the BVF was fine all along, and it's just that after the Duc's 0.65Nm/kg, many bikes feel feeble, including the 0.21Nm/kg ETZ (using the factory spec'd 27.4Nm torque & a measured 129kg dry).

[Kruh]

On riding at full throttle: I guess it depends on the rider & on where you ride. If it's in a city or built-up area, true, rarely full throttle, and I keep to the indicated speed limit. But on twisty country roads, which is what I do mostly, if I ride alone there is a lot of full throttle on the ETZ, or actually with any bike.

That is interesting.
Perhaps some of that may be due to differences in powerbands between the TS and ETZ. With the ETZ being more oriented towards the upper rpms.
It is quite likely I just don't ride it hard enough, but if I'm recalling correctly, my TS seems to rev out already at around 3/4 throttle, even on twisty roads.
Perhaps, I'm just being fooled by the tachometer or worn out engine components, making the bike vibrate more than it should.

[Puffs]

Yes, I just had a look at the power bands of the TS250 (MM 250/3), ETZ250 (EM 250) and the ETZ251 (EM 251), and the TS does seem wider. However, the TS peaks at 5750rpm, the ETZ's at 5500rpm.

For what it's worth, see attached. These are confusing graphs, and particularly for the 251 inconsistent with the text (where they say it does 21hp=15.5kW).

I do not have a rev counter (no cable), I shift gears when I feel the engine roll off the power curve - is that what you mean by 'rev out'? I would expect that if you'd give full throttle, you'd reach that point quicker?
The Doctor's advise was: brake later, and open the throttle earlier. Wide open, I think he meant.

ETZ251 (EM 251) performance chart.jpg (27.16 KiB) Viewed 1622 times

ETZ250 (EM 250) performance chart.gif (40.49 KiB) Viewed 1622 times