OKO carb for ETZ251

[alexxx]

I just cannot get rid of the surging and poor tickover, no air leaks, carb gaskets ok
Got to be mainly down to the carburation.
I put the bike in the shed and did not even MOT it this year as it was too annoying anytime the throttle was closed so will try a new technology carb.
The surging really spoils this good bike, were they like that from new?

Regards

[Andy_C]

Alexxx.

Sorry to hear that you have parked the bike up - as you say they are good little bikes.

Just one suggestion, earlier this year my bike went through a phase of not wanting to tick over, surging like mad and generally running very poor.

After much head scratching and messing around, the problem turmed out to be electrical - I found that a lot of the push on terminals were very loose, particularly around the rectifier & regulator box.

I pulled them all off and closed them up with a pair of pliers before re fitting them - no problems ever since, it has been running better than ever.

Also have found through experirnce that if the battery is not holding charge it can have a similar effect.

Might be worth a look if you feel inclined.

[alexxx]

Thanks Andy,

Will check over the electrics as you suggest.
I fitted a new Yuasa 9amp battery and the bike has electronic ign as standard, it could be a combination of lots of small issues.
Happy to fit a new carb to see if I can improve over the original design.

Regards

[DAVID THOMPSON]

I found that a lot of the push on terminals were very loose,

my 660 traveller has this problem and the rt 125 has had some loose ones also

Dave

[Puffs]

I understand surging to be the following: you close the throttle (for instance while going downhill), and for a while you then get no ignition, and then: BANG one cycle ignites (and the bike surges forward, or at least the RPM hikes up), a few more non-ignited cycles, and then one more BANG, etc.

My ETZ251 does that too, a bit. But it doesn't really bother me, or I pull the clutch.

I think the cause is that you close the throttle. Because of that, very little air/fuel mixture can enter the engine, resulting in very little scavenging (leaving exhaust gasses behind in the cylinder), and much lower pressure above the piston @ TDC. The lower pressure means that the gas has not been compressed much, so that the temperature is lower. (Adiabatic compression is required to heat the mixture in order to let it ignite properly - Diesel engines rely on that alone, but also MZ 2-strokes need the mixture to be heated & compressed for proper ignition.) This lower temperature & pressure, but particularly the abundance of old exhaust gasses & little fresh mixture, results in the mixture not igniting. And then, after a number of cycles, the gas is clean enough, and it does ignite, once. And on it goes, until you open the throttle.

I think the problem is that 2-strokes are gas pumps, but they do require a minimum amount of gas to work. (I mean 'gas' in the sense of one of the four fundamental states of matter, not 'gas' in the sense of fuel.) Surging also occurs in modern 2-strokes, with all singing all dancing top of the line carburettors & CDI ignitions.

One solution would be to make it idle really very low. As a result, you get almost no mixture entering the engine when you close the throttle, and you get virtually no surging ignitions. The cost of that is that if you wait for the traffic lights, you may have to control the idle with your right hand. Of course you have to get the jetting (in this case particularly the idle jet & needle + guide) spot on, and the low idle RPM solution only works well if you have an oil pump. Also, I'm not certain you can actually do this with the standard BVF, where you set the idle using a bypass circuit, rather than by the lift of the throttle valve, as is more common.

Interestingly, my Jawa 634 has no oil pump, and a proper idle, but I cannot remember having any significant surging issue there. But the engine is quite different, in that it is a long-stroke engine, with a crankshaft that is much heavier, certainly compared to the 175cc per cylinder (the surging BANGs occur per cylinder, it's a 180° twin).

When I was slightly younger I had a 2-stroke DKW car, which had an automatic freewheel setting particularly to go downhill. I cannot remember if it surged, but the freewheel might have been to combat surging. Alternatively it might have been to ensure lubrication, although it did of course have an oil pump.

To cut a long story short: I don't want to discourage your experiments with the OKO carburettor, but the possibility exists that won't solve it either.

[parrbd]

I have found surging is cured by running a rich pilot setting on all the types of two strokes that I have owned.

[Blurredman]

Also, I'm not certain you can actually do this with the standard BVF, where you set the idle using a bypass circuit, rather than by the lift of the throttle valve, as is more common.

Interestingly, only the very last production of the BVF used the bypass circuit, the 30n3-1. This was only standard on 251 machines. Previous MZ motorcycles used the throttle valve lift adjustment for idle setting. And before that, as I recall- the idle adjsutment was on a nut adjustment on the throttle tube- Which is still found on the later models, but I think mainly adapted for the use of adjusting throttle lightness.

[Puffs]

Ta for that info!

It is a significant point: that 30n3-1 as found on the ETZ251 has an idle with the throttle valve completely closed, and all flow going through a bypass circuit. Probably on the OKO you control the idle by the lift of the throttle valve, and maybe you can get rid of the surging with such a carb.

[alexxx]

Thanks folks,

Helpful as always.
My bike does have the 30n3-1 carb.
Happy to try swapping it out, does not cost very much and if no difference I can use the carb in another bike sometime.
Will post on here once I have fitted and tested it.

Regards

[Andy_C]

My ETZ 250 is running a 30-n3-1 carb.

As I said in an earlier posting

"when I first had my ETZ is used to buck and stutter and was very unpleasant on the overrun, it is pretty good these days. I spent a lot of time fiddling with carb settings - mine has a BVF and it is happiest with 2.5 turns out on the airscrew, and 4 turns out on the bypass screw."

It is pretty sensitive to settings, air screw in too much and the pickup is awful, bypass screw too far in and low speed running is poor - seems to 4 stroke a bit.

It is a juggling act, try using my settings as a base and go from there, but only adjust 1 screw at a time and in small increments 1/4 of a turn is good.

[Puffs]

Indeed, start from that as a base, as I understand that Pilot: 2 1/2 out + Bypass: 4 out is the standard setting for the 30n3-1.

As mentioned in my 20 Oct 2018 post above, on my 251 surging on the downhill overrun (with the throttle closed) is also a bit of an issue. It also has the original 30n3-1, and initially I tried if the needle position (set at #3 from the top of the needle) made a difference. I tried one position up & one down. For now I left it at #4, which is slightly richer than before. However, I'm likely to go back to #3 as it now tends to 4T at 1/3-1/2 throttle, depending on the load.

I found the needle to influence surging only very little - it still surges, and it did similar in all 3 positions. Maybe if I reduce the idling RPM (so that less mixture enters the cylinder when going downhill with the throttle closed) I can reduce the surging, I have yet to try that.

But I first wanted to try what influence the reed valve I had made for it a long time ago (see page 1 of this thread) would have, so I installed that:

Reed valve installed.jpg (35.45 KiB) Viewed 65 times

Other than installing that, no adjustments were made to the carb (or ignition). Riding it with it showed:
- idle increased significantly (from about 1300 --> 2000)
- it still surged a bit, but less than before
- as said earlier, MUCH stronger in the lows, and you can give it full throttle from, say, 1500RPM onwards. Without the reeds you best not give full throttle below 3000RPM, and at low revs it doesn't pull anyway.

But the bike currently has a pretty much standard piston, with the standard input skirt lengths, so you don't get the high-rev benefit of extending the inlet-open time reeds would normally bring. Like this, in the highs you loose responsiveness, and actually power. Though top speed is still more or less the same, it doesn't pull as willingly as without the reeds, if you keep it in it's power band (which requires more shifting).

And the other thing is: the reed assy has been used for some 4 years, say 12-15k miles, and the reeds are tired. Letting them rest for about 15 years hasn't helped. They are both bent open, and on low rev/full throttle you hear a loud clacking when they close. That's why I took that assy off at the time, I suppose, at any rate, that's why I took it off now.

To be continued...

[Puffs]

The first thing I did after that, was return the needle to it's previous position. That's not actually quite #3, more like #3½ (in-between #3 & #4). This is what it looks like,

Org = preferred position (~#3).jpg (16.86 KiB) Viewed 65 times

and the reason it's like this is because I had lost the original clip into the engine a long while back. The steel clip it has now is so small that hopefully it won't cause any damage. The original did cause damage. Below the steel clip is a 2mm nylon bush to stabilise the needle, and I hope that bush won't cause any damage either in the event something happens. One can hope.

Looking at the drawing of the 30n3-1 carburettor in the Haynes manual, there is an 'O'-ring on top of the carburettor's throttle housing, in the lid that screws on. I didn't have that. It cannot leak much across that very fine thread, but I had an 'O'-ring that fitted fine, so I installed that. And adjusted the throttle cable for the lid coming up a bit. Then I found that the 'O'-ring made no difference, so I was back to where I started - but left the 'O'-ring in.

Next I focussed on the idle adjustments in this carb. Until then I had: Pilot 2½ out, Bypass 3½ out, idling at about 1300RPM. That ran fine, but it surged. I then tried to lower the idle (against the idea to reduce the amount of mixture it gets on the overrun), and I arrived at:
Pilot: 1¾ & Bypass: 2¾, idling at about 1000RPM. It ran pretty well, but cold starting was a bit more difficult. I did about 70 miles with that, and it still surged. Initially I thought it was a bit less, but probably that was just wishful thinking.

So then I went back to the 'Standard' setting: Pilot: 2½ & Bypass: 4, idling at about 1400RPM. The take-up is better than with the low idle, and for the surging it does not appear to make any significant difference. The bike runs fine, but it still surges going downhill w/ the throttle closed.

Not sure where to go next. Maybe another carburettor (in which you can close-off the flow better), maybe back to reeds? Or maybe just pull the clutch.

EDIT:
Not satisfied with this, I figured: you should be able to reduce the surging by reducing amount of mixture. So I went back to the idle settings, and lowered the Bypass. If you do that, the RPM reduces, but also it starts to run poorly & 4T, so then you also close the Pilot a bit. It then starts to run well again, and the RPM increases a bit, but stays lower than before. I went down to the lowest it would still run, which was: Pilot: 1 & Bypass: 1⅞, idling at about 600RPM. By then the charge indicator lights firmly, of course, but it runs evenly & doesn't stall. Might even be 500RPM, can't say for sure as the needle wiggles a lot (cable damaged a bit).

Anyway, at these low settings, for all practical purposes the surging is gone, so let's try that for a while.

[Puffs]

Following on from this, I too tried one of these PWK clones, a Koso PWK32, as detailed in viewtopic.php?f=20&t=10290&p=51251&sid=8a66b7fd0117aafef4b03ba2f25ddcef#p50780. When I wrote that, I had not done to many miles with it yet, and my first impression was that I thought to see some gains at the lower end, though the top speed had not benefited. A pic:

Koso PWK32 mounted.jpg (21.08 KiB) Viewed 65 times

(It also received clamps on the manifold & air box side.)

I always find it difficult to judge if something like this, like another carb, brings any real benefit. Initially you have just bought & installed that shiny new gizmo, and by the nature of that, you/I tend to have a more positive view on the matter. New is always better, for if it isn't you wasted your time & money. And also, when judging differences, you have to compare how the thing runs now, with how it ran several days back. I find that difficult. It is a subjective impression, which I find difficult to remember. For top speed, I use one particular stretch of road, shielded from wind, so that is a reasonably objective comparison.

I still haven't done many miles with it, but a few hundred, and now also in nice weather; I think enough to judge. Now I think the gains I saw at the lower end were actually due to me raising the barrel (by 1mm), which I did just before fitting the Koso. And top speed-wise, there is not really any difference between the two carbs either, and neither is there in the way the engine runs. With both carbs there is some minor surging on the overrun, but nothing significant & it doesn't bother me.

So now that the honeymoon period is over, I thought: let's try the original BVF again, and get a new appreciation of the differences. That is easy, as I had made a dedicated intake manifold for the Koso, and kept the original one for the BVF. Put the BVF back, and I now think there is actually very little difference between the two. As swapping is easy, I'll probably go back in summer to give the Koso another try.

I think the Koso is nicely made (I like the chromed slide & carb body), it's good value for money and it works well. There's nothing wrong with it, but it does not really bring any significant improvement either, and for originality I'd prefer the BVF.

[Blurredman]

And there you were saying that originality doesn't mean best.

I have no experience of Bing carburettors, but I've read alot into carburettor swaps on these bikes.

But out of the original line available with these bikes, I still think the BVF 30N3-1 has got to be the best BVF carb..

You mention performance, but alot of people find fuel consumption to be better on other carbs than with the originals. Did you notice any mpg increase- or even decrease? If it decrease, is this potentially because you were testing the bike to the limits and were feeling throttle happy? What if you rode it like a normal person on a commute?


Perhaps what you should do is map out a track route. only about 10-20 miles of a nice circular road. One with some city traffic, some with country lanes, maybe a part with a nice place to do a full throttle run. Do the route several times find out what gear you need to be in or where the rpm's are at specific intervals... Then swap carbs.. Can the new carb seem to climb that hill a little better than before? Did I previously need to change down to be able to take that corner without labouring the engine?

You could even fill up the tank for each carb and measure consumption...


Or maybe that's just way too much effort

[Puffs]

Bing is another fine maker of carbs, with many years of experience. West German, I believe. Around '75 I put a Bing 54 on my CZ 980, to replace that Jikov; that Bing is still on. Not quite in the same price range as this Koso though (around £14 for the Koso if I remember correctly).

For reference a few pics of the Koso:

Koso PWK 32.jpg (42.23 KiB) Viewed 60 times

Koso 2.jpg (29.53 KiB) Viewed 60 times

I have no idea how this carb influences the fuel consumption for this bike. Sorry, not my focus, not even when I used the bike for commuting.

Edited:
Thank you for the guidance on how to compare. Yes, I said "I always find it difficult to judge if something like this, like another carb, brings any real benefit.", and thereby invited it; and I agree, what you said makes sense.
I went back to the BVF to once again perceive how that one performed, and the difference, if any, is not very obvious. In the end, it's just a subjective impression I'm after: with which one do I like it best? I will see what I'll do, and may come back at a later date.

Thinking a bit more about fuel consumption & a different carburettor, I'm not sure a different carburettor should make any significant difference. True, there are differences between the Koso and the BVF: the low-speed circuits are different, and the Koso is essentially a flat-slide carb (which enhances the venturi effect at low & mid slide openings). But the way the fuel is nebulized is very much identical. That fuel atomisation could make a difference; an injection engine can run cheaper (also without ECU) because the combustion process can be more effective.
Fuel consumption has all to do with how efficient the engine is (and of course how you ride). Things like port timing, port sizes, swirl, fuel atomisation, CR, ignition, exhaust, ..., all influence that, but none of those change in any way. And as long as both carbs give the same fuel/oxygen ratio (= both are jetted correctly), the engine should get the same amount of fuel.

Another (small) difference here is that the BVF is a 30mm carb, and this Koso a 32mm. If the rest of the breathing process will allow that, at full throttle that can cause more mixture to be aspired through the Koso, which is also when also that 'Power Jet' comes in action. So at full throttle the Koso should provide a bit more fuel. BUT: exactly that I tested quite objectively, it doesn't go any quicker (or slower), and also the plug looks very much the same.

Fuel consumption-wise I do not expect a significant difference, but I have not measured it with either carb.