EtZ 250 Carb Question

[nice2day]

Just a quick question please. I am trying to tighten up the carb on the manifold but it remains loose. I have memories now of the carb having a white nylon sleeve around the stub to make it a touch larger in diameter... Is this correct or do the carbs fit bare, straight into the manifold
The carb is the later BVF 30n3-1 ...If it does require a sleeve...I have lost it and already spent an hour looking for it or something that does not exist anyway....

Les

[Guesi]

It is simple.
One of the parts is worn out. Fixing an aluminium carb into an aluminium manifold will not work forever...

Especially if you try to pull out the carb from the manifold by turning the carb left and right until it comes off.
This way one of the parts will wear out ...

[nice2day]

Hello Guesi, thanks for replying. I think you are quite correct. I realise there is no room for a sleeve but somehow the fit has become worn on BOTH the manifold and the carb stub. I have to say though that usually I am very impressed by the East German technology but this method of holding a carb and thinking it would be perfectly air tight is flawed as it relies on a sliding fit and the compression of a thick solid tube of cast aluminium via the 2x slits and 2x bolts. Cast aluminium is very likely to crack if it is made to flex and the 2x slits can let air in around the side of the carb stub.
Anyway I have managed to get the carb to hold by sawing the slits so they extend a touch further towards the engine. The extra length of the slits allows a little more flex to occur and after tightening the bolts to extreme pressure to a point they are just short of snapping I could get the carb to hold fairly tightly. Hopefully the assembly is air tight, but I will test when running.

The reason why there is wear is because the bolts were perhaps not tightened sufficiently from new which allowed engine vibration to wear the fit between carb stub and the inside of the manifold. You have to remember my engine has NEVER been stripped down before and has only done 7000 miles so should not have been unduly worn or damaged. Thanks again Guesi.

Les

PS...I have just though that I could have used a thread locking compound around the carb stub to ensure a gas tight fit...I'll see if it is needed when I complete the rebuild.

Les.

[Guesi]

Or you do it like that :

[Puffs]

Les, if I remember correctly this topic has come up a while back too. As said, a metal-to-metal fit is original. Yes it can wear if not tightened properly; I have been lucky (apparently) as mine is still fine (btw, I fit the carb with some silicone grease). Wanting to save the original manifold, I made another manifold to test the PWK, there using a rubber muff like in Guesi's pic. A rubber muff has the further advantage of better isolating the carb from vibrations, which wear internals like needle/guide/slide/fuel valve. But of course, if you adapt it for a muff, you destroy the originality of your manifold.

This wear and other things you mentioned may cast doubts on the reliability of the mileage indicated? A counter is easily replaced.

I'm not sure tread lock compound is the right way to go. You'll glue the 2 together. Of course you can take the lot off in one go, but still. I'd first just try with a silicone sealant. If that doesn't work, due too too much clearance, I'd try to open the manifold a little, and then glue thin copper in it (with epoxy). Of course making sure you'll have a good fit afterwards. The carb mouth may also need some work first.

Good luck!

[Blurredman]

I use standard green grease to fill the gaps and to reduce possible air leaks.


You know there is talk in the official manuals about a 'packing' that goes between the two surfaces...?

I believe it's made out of a sort of paper and it is supposed to be renewed if required but the bikes work nicely without them which is probably why all the bikes have them gone a long time ago.

[Guesi]

Can you tell me where this talk about a packing is exactly ? I never saw this in any book.

[nice2day]

Hi Puffs Thanks for your comments. yes I would prefer to keep the bike as standard as possible so ultimately I could buy another intake manifold. I think you are right about using "ThreadLock" and your suggestion of using silicon sealer is a good one and will use that if need be. As regards mileage the total lack of wear anywhere in the engine seems to back up the promised mileage that the seller said and the speedo shows. For instance, the piston does not have a single mark along the side of it. The thrust side of any piston usually has some evidence of contact but the piston looks new as does the bore. All other wearing parts look pretty well new too. I simply think the carb became loose early on in its life and vibrated the carb and wore the manifold ...of course factory machining errors and summing of negative tolerances could be the culprit too.

Regarding Blurredman and Guesi's question about packing. I think the term "packing" is used all the way through the official manual but it is a bad translation and we would normally use the word "gasket/s" ....The carb section is talking about the inlet tube gaskets I think.

Les

[Guesi]

But these daskets are bewtween the manifold and the cylinder, not between carb. and manifold.

And by the way, silicone sealer contains vinegar (which is an acid ).
So if you want to put acid on your aluminium material, go on...

[nice2day]

Hi Guesi...I know that!.... but I was answering Blurredmans question that he THOUGHT the manual suggested fitting packing around the stub but I explained he had misread the text or forgotten what he had read...we KNOW there is no gasket or PACKING around the carb...hopefully you will understand my reply now?

Yes some silicon sealers do contain acetic acid but not ALL of them do. Some are Non-acetoxy. Bare in mind there are MANY engine sealers or gasket substitutes that are silicon based...

Les

[Guesi]

Hello

I understand.
Generally I think that sealing the carb with the manifold will not solve the problem of the bad connection in the long run....

If the carb cannot be tightened good then one or two of the parts are damaged. And will be also after sealing it.
The danger of additional air and overheating of the engine will remain ....

[Puffs]

On the mileage, Les, you had the thing apart so you know pretty well what it's condition it's in. Then there's some other cause, as you say.

On sealant & acidity, there are major chemicals companies like Loctite (a subsidiary of Henkel, I believe) that supply silicone-based sealants especially developed & marketed to seal on all sorts of joints, explicitly including aluminium, over a temperature range from -50°C to +250°C, and resistant to all common fluids. I have been using Loctite Blue for quite a few years now, mostly on aluminium alloys as used for heads, barrels & crankcases, as it says on the box, and never saw any ill effects. Vinegar or not, it works for me.

It remains soft and develops a poor bond with the metal surface (good enough to seal, but still easy cleaning on disassembly). If the carb's stub is not tightly fixated inside the manifold, after tightening the 2 screws, then it will continue to move under vibrations & continue to wear; then that's no good. That's what I meant with 'too much clearance'. If on the other hand the two do get to be fixated properly, then the silicone sealant will help prevent any leaks & you're fine. Seeing the work Les has done I'm quite sure he'll understand & sort it out.

Btw, silicone grease does not at all smell like vinegar, it is a clear, thick, sticky grease, high temperature resistant (I also used it between coolers & CPU's). I hope it doesn't dissolve in petrol, that's why I use it here, but I'm not sure of that. Anyway, as I have it.. If I wouldn't have it, I'd just use another grease (as it's a tight fit).

[Guesi]

Please forgive me clear words, but all this stuff with silicone or other sealing material where no sealing material is recommended is a botched job.

The guys from MZ knew what they did and why they did it this way.

I remember in the eighties MZ was a cheap bike for us (we called it " Kawasaki for students" ).
So we tried to keep these bikes running spending as less money as possible.

But these times are gone...
Nowadays MZ is a respected Oldtimer here.

There are so many parts , also second hand, available for these bikes.

So please change the worn out or damaged parts and donß t try to fix it with any "russian methods" as we say here .
Often the result of these methods are more expensive than changing the parts in the first time.


And believe me, I don´t say that because I sell parts for these bikes, it is because I drive these bikes for about 40 years now and I have seen all of these things in the past...

[Puffs]

While bowing to your seniority in MZ matters, I'm politely pointing out that Henkel is German & Loctite is American.

Grease is no sealant, and is just used to aid fitting & later removal, as it can be a tight fit. I'm surprised to hear that you think using a bit of grease makes it a botch job.

We all know the risk of a poor fit is aspiration of un-metered ('false') air, particularly at almost closed throttle, which could cause the mixture to be lean (in addition of course to the wear described earlier). So I think we agree that joint should seal adequately & reliably. Depending on the overall fit, a bit of silicone sealant can help that; it was designed for exactly such purposes (in Düsseldorf?). Of course if there's too much clearance you won't get a reliable fit & something else is in order.

[Guesi]

Hello

I never said that you should not use any grease.

And I don´t care from which company or country a product comes from

And I wrote :

all this stuff with silicone or other sealing material where no sealing material is recommended is a botched job

So where sealing material is recommended by MZ you can use whatever you want.

But for example MZ never recommended any sealing material between cylinder and cylinder head...or between manifold and carb.