TS 250 PWK carb

[Kruh]

I fitted a PWK 30 mm carb on my TS. I've ridden the bike with this carb for over 3000 km and I'm pretty happy with it.
Specificaly this is a OKO replica.

You are probably interested in the settings and mounting, I sure was when I looking around but didn't find a whole lot.

My settings:
Pilot jet: 42
Main jet: 116 (Dellorto jet); 122 Keihin equivalent
+power jet (not sure which size)
-needle in the middle notch, its the same that came with the carb (no writing on it)

I mounted it by cutting the original flange, specifically just the part where the original carbs inserts to. Than I just filed until a got a smooth and round flange. Onto which i put the included rubber coupler.
Turned out great. Imo, better than the original. Should seal better and minimize vibrations to the carb.





Being that this is a cheap replica I did run into a issue.
The thread on the float bowl and drain screw is not made well. So its easy to crossthread the drain screw and strip the thread. Which is exactly what I did.
Good news is that these carbs are very cheap. For the price of a bowl and drain bolt I could just as well buy a complete carb.

I really like this version of the carb:



The only issue is that those verions are available only in 33 mm (smallest size)

So, my question to you is:
Is a 33 mm carb too big for a stock TS 250?

If I go with that carb, I would enlarge the intake manifold from the stock 30 mm to 33 mm. So the manifold is not a bottleneck.
My goal with this isn't necessarily to get more power. Although a slight bump wouldn't hurt.

[Puffs]

Hi Kruh, thanks for sharing that!

Your post is ahead of a post I intended to make, on fitting a 32mm Koso PWK to an ETZ 251. I understand both OKO and KOSO produce replicas of the Keihin PWK, here's my 32mm KOSO PWK clone:

Koso PWK32 mounted.jpg (21.08 KiB) Viewed 72 times

They probably look quite the same, only mine says 'KOSO' where yours seems to say 'OKO'. I have only very recently fitted the carb, and not done many miles with it at all. I'll still do a post on it in spring time, maybe summer, when I have more experience with it.

For the record, my current settings:
Pilot = #43,
Air screw = 2¾ turn open,
Needle Jet = original,
Needle = original @ middle position,
Main = 1.40mm (which is slightly richer than the #138 Keihin originally included).
The Power Jet is not marked and I cannot measure it, but it is 0.35 < PJ < 0.85 mm, so pretty small. Note that the original Keihin PWK does not have that Power Jet.

A number of jets were included it the package, but I found that the biggest (said #138) was slightly lean:

Plug with #138 MJ (fair test).jpg (25.36 KiB) Viewed 72 times

For now I landed on a 1.40mm one, giving:

Plug with 1.40mm MJ (fair test).jpg (30.38 KiB) Viewed 72 times

While I'm showing these plugs, for me most important is the subjective impression of how the engine responds. And I do have the impression the low & mid range pickup is stronger than with the original 30mm BVF. This is not due to the size, as in that range the throttle is only partially open. But: as said, my experience so far is quite minimal, maybe 25 miles, and all at temperatures near freezing. Because of those low temperatures I may choose a smaller Main Jet under normal conditions, but for now I'd like to err on the side of caution.

I have had no issues with the carb, other than that the engine side of the rubber booth was way too small (smaller than the venturi size). I like the carb, specifically the chromed bore & flat slide. I also like how easy you can access the main jet, via a big screwed cover (sealed with an 'O'-ring) just below it. And I also prefer the choke on the carb, rather than via yet another cable coming from the handlebar. On that power jet, the jury is still out, I'll say more about that later.

Looking at your picture, I suppose that was during fitting the carb, as there is no fuel line or connection to the air filter.

On your question "Is a 33 mm carb too big for a stock TS 250?", my views are as follows. Like the ETZ, the TS 250 originally also has a 30mm carb. Increasing the venturi size alone will not give much improvement, in fact at full throttle the bike might run worse (because the flow velocity reduces, the venturi effect is impacted and the jetting might be less accurate). The entire engine, meaning the porting (size + timing), intake & exhaust are designed in line with the original 30mm carb size. In principle you can run a bigger carb on a 250cc 2T (I believe up to 40mm have been used on race bikes), but you will only really benefit from a bigger carb if other parts of the breathing are similarly modified. It is not only the intake manifold that requires work (a dedicated one on mine, to retain the original manifold for the BVF), but all ports in the barrel. Maybe make a few extra overflow ports too, and certainly exhaust port work is required, and of course also the exhaust itself (a tuned expansion exhaust?). Then you really enter into the tuning-for-speed area, and while these engines (ETZ & TS alike) can be tuned well, and have been used in racing, one can wonder: Why? The cost will be: losing originality, higher fuel consumption, louder & less reliable, but yes, more speed (at least if it is all done well).

Yes, I put a 32mm carb, and while I think I see some gains at the lower end, so far it does not look like my top speed has benefited. As said, the jury is still out & I need to get more experience. Note that my objective was not to make this bike faster.

Another consideration is when you have an oil pump: the pump cable assumes a carb slide travel of 30mm.

Anyway, buy a bigger carb if you like to try it (as I did), but if you want to achieve real performance gains more work is required. Of course, any aftermarket carb will reduce originality.

[Kruh]

The bike smokes a bit under heavy acceleration. Should be normal to some extent, but it seems a bit too much to me.
A more useful fact; when I ride, on the open road, above 100 kmh there is visible smoke in the mirror but not that much. That is riding at about 1/2 throttle. When I go up to 3/4 or more there is quite a bit of smoke.

Would seem that the mixture is to rich in the high end. But the bike isn't sputtery or "4 stroking". I tried a smaller main jet. The smallest was a 106. Also tried 112 and 118 without the power jet. No major difference. Curently I'm at 116 with the power jet with which it runs well but, as I said it is smokey.

Was even thinking about the crankshaft seal (cluch side) being bad.
But normal riding aorund town and country roads, its all good. I do not see any smoke. Maybe something uphill, but generally it fine.
At idle it also fine.

I'm running 2% premix with fully synthetic oil (Castrol power 1 racing). I assume then it would smoke less than mineral oils.

Is this a somewhat normal thing for MZ. It is a big cc 2 stroke after all. I'm just wondering how a good MZ 250 runs at higher load since most of my experience is with smaller cc 2 strokes.

[Puffs]

At 1:50 = 2% with a modern synthetic 2T oil there should be very little smoke in a well adjusted 250cc MZ 2T. That power jet will only provide extra fuel on long periods of full throttle. This is due to the placement of the exit of that channel in the venturi (assuming it's the same as in my Koso PWK). Also, the main jet has the biggest effect at or near full throttle.

There are a few possible explanations for a very smoky 2T engine:
1) Maybe a leaking oil seal at the clutch side causes the smoke + the carbon particles shown elsewhere? Remember that at the ignition side you found it leaking, possibly due to corrosion or crud on the shaft. If this bike has been standing for a long time, the clutch side might be same. Without a vent in the gearbox volume, the pressure there increases when the engine heats up, and it might push oil into the crankcase. Any change in the gearbox level? (Might not see it, as only little will already smoke a lot.)
2) The air/fuel mixture could be rich if the fuel valve in the carb leaks, so that fuel drains directly into the intake. I've had this once with an old-fashioned valve (with the pin directly on the float, so without the spring your carb has) when the engine vibrated strongly (not an MZ). It seems improbable that is the case here, but if you notice excessive fuel consumption it might be it.
3) Too rich air/fuel mixture due to rich jetting. Optimum performance as well as low smoke requires proper jetting, not too rich. The risk of course is that if you run it too lean, it may run hot & seize. See also P. 27 in the Haynes manual:

Haynes ETZ, P.27.jpg (39.43 KiB) Viewed 72 times

While for a seriously smoky engine, this too is not the most likely explanation, certainly not when using a 1:50 oil/fuel premix, a rich air/fuel mix will contribute. For me the first indications on jetting are the way the engine runs & responds to the throttle, but certainly also have a look at your plug. Use all information you can get.

[Kruh]

Yeah, it is quite possible there is also corrosion the clutch side of the crank. Which would cause wear on the seal.
I did check the gearbox vent (the oil cap), the hole is fine.

I'll try smaller jets again and give it a more thorough testing. Just to make sure. Otherwise I'll be cleaning the crank and replacing the seal for good measure.

The bike does feel good and the plug is fine. Its dry without much carbon. The color might be just a litlle bit on the rich side, but that litlle bit could also be down to other factors (as in your attachments)
Also, I didn't do a full throttle plug chop so this is just the general running state.



Also, thank you for all your help

[Kruh]

I got another carb. This time I went with a ZSDTRP PWK30.

Same as any other PWK, but with a few differences; it has a thumbscrew cap, no powerjet and fuel inlet on the left.

Comparing it to my OKO carb, this one is definetly better quality.
Finish is nice and smooth, everything fits nicely, the slide is better quality (the chrome plating), it also has less play in the body of the carb, the ventury is polishe all the way thru, threads fit more tightly.
Especially the float bowl cap. On the OKO the thread is bad: loose fit and crooked, strips easily.
It could just be that I got a bad one. But the ZSDTRP feel better in every way.


The setup now is:
- Pilot jet: 40
- Air screw: 2.5 turns out
- Main jet: 125, a 120 might be better
- no power jet
- needle in the 2nd notch from top, its the same that came with the carb (no writing on it)



The jets are hex, just like the original Keihin. It also come with a second needle, 3 main jets and 2 pilot (maybe it should've been also 3, but I go only 2).
The needles look the same (same taper). But haven't measured them.












How it looks on the bike. I replaced the choke with a cable one to make it more like the original

[Puffs]

Yes they look like cousins, but your awkwardly named new one looks better!

I actually prefer the Japanese-style choke on the carb: fewer cables. Indeed, less original, but if you want original best stay with the original BVF. Which I might actually do, as for me the PWK makes little difference in how the bike runs. Maybe slightly stronger on the early pickup, and broader, that's a pre, but in top speed the 30mm BVF seemed quicker. I'll re-test later. (My PWK was a 32mm, and my bike a 251 with a 1mm raised Maß A = 12.9mm barrel.)

What's your view on any differences in running of your TS? Smoke?

[Kruh]

Well, I don't care too much abot the look of the carb on the bike. The cable does complicate things. But I like how the TS choke lever looks; it fills the space on the handlebar and makes it look more sophisticated i guess.

I also thought it would have an added benefit of finer control of the choke, insted of just on or off. But in practice it makes no difference. It might be more useful in cold conditions.

It runs better now than with the OKO. Its snappier overall and more stable at idle. The idle was a bit troubling to tune on the oko.
After it warms up there is no visible smoke. With the OKO, it would visibly smoke even at idle.
I done only a few short rides, so we'll see how it behaves long term. Especially on faster roads; with more load.

[Puffs]

Nah, I think the choke is just an on-off affair. I admit the original MZ thing is quite handy, but the cable is a hassle.

Good the seal is sorted now!

Plse keep us posted on your experiences with this carb, I'll do same w/ mine (but that will take some time). I'll probably also try without that 'power jet', by simply blocking it off.

[Kruh]

Regarding the extra needle that comes with the ZSDTRP carb. I measured both needles and they are exactly the same, other than the material. What difference does it make, I have no idea.
I also checked the OKO needle and it is the same.


I managed to get a proper ride now.
The jetting was the same as in the previous post, except I raised the needle to the middle (3rd notch from top), just like I used on the OKO.
Turns out it too rich. So I put in a smaller main jet and droped the needle.

The setup now is:
- Pilot jet: 40
- Air screw: 2.5 turns out
- Main jet: 115
- needle: 2nd notch from top

It is interesting that it prefers smaller jets than the OKO did.
It runs very well now. It is very responsive and it feels like it has more power than it did with the OKO.
Also, now it starts first kick every time.

I do have one issue with it and its the same I had with the OKO.
Sometimes when I stop the idle stays high for a bit, until it settles. Once it settles I can rev and it drops normally.
This would indicate to me that its lean, but when I turnes the airscrew in a bit, it was too rich (slugish on the throttle).
The throttle cable doesn't seem to be the cause either.

Any possible culprit, other than just me not tuning it correctly?

[Puffs]

Thanks!

On the needle: it would be odd if all those needles were the same; are you sure there are no differences in the taper? Measure at both ends of the taper, with a micrometer, or maybe just a calliper set to a fixed width & see how far the needles enter.

On the initially faster idle: not sure. I think I had that too on some 2T's. Maybe it has something to do with the engine then being warmer? Lubed better? Obviously you could richen the idle & raise the throttle valve a little more, but if it runs well as-is, why bother.

[Kruh]

I checked the taper as well. It is exactly the same.

[Puffs]

That's a bit daft then - why give 2 needles if they are exactly the same??? But then again, with my Koso I received 4 jets, of which also 2 identical. Ah well.

If you want richer, you can put one in a lathe (or stationary drill) & trim it to your liking...

[Kruh]

After about 3000 km of riding with the ZSDTRP carb, everything is doing great.
There is wear on the slide - some of the chrome has dissapeared. Perhaps too much?
As far as I know, these slides do wear on the bottom edges, even on the original carbs, but the middle portion probably not that much.

ZSDTRP slide left, OKO slide right. (the milage on the OKO is slightly less)



Onto jetting:
I lowered the needle all the way during these summer days and rode like that for a while. But it was a bit lean.
So I went from a 115 to a 120 main jet and raised the needle by one notch (2nd from top). Now there was an issue of stuttering at around 1/4 throtlle.
Compared the throttle valve cutaways on both carbs, and the OKO had a slightly larger one. So I tried that slide to lean the mixture at that throttle position. And it was an improvement.
So I copied that on the other slide, because the OKO slide has a lot more play in the body of the carb. (I couldn't find any specific dimensions on the original keihin slide cutaways)
Went from 3.9-4 mm (don't remember excatly) to 4.2 mm.




Spark plug:




The setup now is:
- Pilot jet: the same as before
- Main jet: 120
- needle: 2nd notch from top

[Puffs]

Thanks for sharing your experiences with these carbs Kruh, and all these details!

From my side I have nothing new to share, I removed the Koso after a relatively short mileage; not for any failure or so, but just because the original BVF is fine too. Maybe more surging on the overrun.