MZ ETZ 250 Started yesterday

[nice2day]

Well I started it yesterday. The first time it has run since 1994. Although it started easily I did not have it running well for quite some time as it would not run without the starting jet activated. I think it was because I put in a very oily petrol mix (probably about 15:1) so I could run it as the oil pump needed to fill the delivery tube and I wasn't going to have the mess of trying to fill it manually. I had tried walking the bike around in gear with the plug removed but the delivery rate even at full bore is painfully slow and exhausted me.
I have now diluted the petrol down now and I have managed to get it to tick over reliably but I'm still not completely happy there aren't any tiny air leaks around that loose fitting carb stub.
What was slightly disappointing was hearing that clutch rumble sound, I had completely forgotten that spoiling characteristic. It's not that bad and obviously disappears when the clutch is pulled. Perhaps I could have got the clutch thrust washer a touch tighter if I had been a bit more patient and tried to buy a thicker washer. Having said that, many years ago a friend bought a new ETZ251 and I'm pretty sure his clutch sounded just as loud if not louder than my one as it is now. Before I knew about MZ's, I was convinced there was something very seriously wrong with his engine and his dealer was just fobbing him off when they said it was normal..."they all do that sir" .
Anyway just the seat to renovate and it's all finished...I have to say this complete nuts and bolts restoration has been a longer slog than I anticipated, I'm sure I could have done 2x British bikes for the same effort.


Les

[Puffs]

Congratulations Les!

On the oil pump, I have taken mine off as I'm not sure that some of the problems I had in the past were actually associated with it. And I don't mind pre-mixing (I do 1:40 nowadays, for the 251).

Hope you get it running well soon!

[Blurredman]

So where are the pictures Les?


Yes, my 251 also has a very loud clutch rattle- the 250 isn't nearly as bad, but can still be heard a little.

At least it won't really do any harm..





It can be difficult to get the bikes running perfectly to your liking sometimes. There are many variables, and they seem to change. If there is an air leak, then surely that would represent itself with a hanging idle, or maybe slightly increasing idle. This can sometimes be a sign of too leak pilot mixture, but in my experience that makes the bike slowly die.


You have a 30n2.5 or 30n3? 30n2.5 pilot screw is 'about 1 turn', and NOT 2.5 turns as Haynes says..

[nice2day]

Thanks Puffs and blurredman. I'll only post some photos when it completely finished and maybe a Youtube video too. Although it doesn't need much, the full completion will be some weeks off as trying to find the time to do the last details will difficult subject to my current DIY workload.

Regarding the clutch rumble...have you ever noticed that if you pull the clutch and release it sometimes the noise is less?...I reckon this occurs because the friction plates change location and the clutch as a whole is less unbalanced and does not wobble or vibrate as much...certain alignments create less imbalance if you see what I mean? However, the manual says it is because of the helical drive transmission creating side thrust ....and I can see and understand that too. What happens is that at tickover the rotational speed of the crankshaft varies quite considerably over the 360 rotation...there is a sudden jerk when the engine fires and a slight slowing at the compression stroke. This variation of speed at tickover creates varying axial (in and out) thrust on the helical gear causing it in effect to tap sideways against the thrust washer; the bigger the gap the louder the tap...it is nothing to do with the needle or thrust bearings as some people think. Once axially loaded by the thrust of lifting the plates by clutch operation or by driving the bike in gear, the axial slack in the helical gear movement remains loaded and no noise is created...It makes me wonder if the engine designer ever considered setting the helical gear cut the other way round so the thrust movements went the other direction...it might have created less noise..... we will never know of course.
Les

[Blurredman]

There is also the two washers- thrust washers either side of the needle bearing- the thinner one is on the crank side if I recall? Did you notice this? There is also a direction in which they are to be assembled, as well..


Yes- I notice that the clutch rumble (on my 250) or rattle (on my 251) can change.. I think my 250 is much tighter and better clearanced there, because sometimes it is gone yes.

Have you noticed that the rumble/rattle can change in severity depending on also the type and weight of oil used, and also when the engine is cold vs warmed up etc etc and all those other variables?

[nice2day]

Hi blurredman...Yes I did make sure the thrust washers were put in the correct way round. I was also aware of the fact that the clearance should be between 0.1 and 0.05mm ( 4-2 thou) I attempted to measure this and thought it was OK at no more than 0.1mm but having said that it is difficult to measure plus the fact that at that moment in the rebuild I was getting fed up and wanted to get it all screwed back together and I didn't want a £15 postage charge for buying 1X washer. The manual says that the gap must NOT be TIGHTER than 0.05mm clearance so as this is still a gap, they are actually admitting that some noise will always be there but the bigger the gap the louder the noise obviously.
As far as I can tell the hotter the engine the louder the noise. I can only assume this is because the oil in the gearbox transmission heats up and gets thinner. I don't think its anything to do with parts expanding as all the parts involved in the noise production are the same type of steel so any expansion will be self compensating since as the crankshaft end gets longer the thrust washer and drive hub expand too and reduce the gap at the same rate and bear in mind the small temperature increases involved will cause only minute size increases. So it is the oil thinning out and the thicker the oil, the less noise is produced. The noise of the clutch rumble is quite low on my bike but there IS a difference when the clutch is pulled. Only direct comparison with other bikes will reveal how noisy or quiet my engine is.

Les

PS: One way the slap could have been nulled out was to have designed the hub to have a large powerful spring instead of the innermost thrust washer (perhaps a very thick spring (perhaps only 1 1/2 turns) sandwiched between 2x thrust washers. The spring would apply constant pressure so the clutch hub would have no slop at any time.

[JawasandMZs]

Inlet side of carb sucking in air ... there's a handy lip inside the inlet stub where you can put an O ring. A set of assorted generic O rings cost about two quid. Order a specific single one often & it costs something silly like a fiver!

[nice2day]

Hello JawasandMZs. I'm pleased you mentioned this idea as I also thought about doing the same thing, it would be a very simple and highly effective fix.
I gave this lots of thought a few days ago and maybe I'm worrying too much but I considered that the rubber O ring has a slight chance of distorting inwards and getting sucked down the manifold stub into the engine as there is nothing to really hold onto it. As you would know AMAL carbs use an O ring in this position but they have a circular groove that the O ring sits in so the O ring is trapped in position. There is enough metal on the stub of the carb to machine a groove if you had the necessary milling equipment and this would solve the problem. I then considered that instead of an O ring you could use a thick fibre or hard rubber washer the exact size of the lip inside the manifold. Being so much stiffer than an O ring it would resist being distorted by the suction and because of this stiffness would not be able to get sucked into the induction tract as the diameter would be too big. I even thought that some enterprising person could get a batch of steel washers coated in soft rubber made up and sell them to MZ ETZ owners. This would, I am pretty certain, cure any air leaks on this carb set up....as long as the carb was pushed in hard against it and clamped up.

Anyway I'm going to do some searching on Ebay
LATEST: These look potentially ideal.... temperature and chemical resistant: https://www.ebay.co.uk/itm/2MM-THICK-WH ... Sw0blbcst-

I'll have a measure up and buy some

Many thanks again
Les

[nice2day]

Just before I turned the computer off another idea came into my mind.
If you wanted to pursue the O ring idea rather than a washer, here is a safe way of implementing it.
Choose an O ring with a thickness of about 3mm and an Outside Diameter of exactly the manifold tube diameter at the lip position.
Then carefully file a 2mm chamfer at around 45 degrees all the way round the edge of the stub of the carb. When pushed all the way up to the O ring, the 45 Deg chamfer would hold the O ring securely in place and at the same time squash it and seal perfectly at the perimeter....perfect.

Nighty night!

Les

[Puffs]

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[nice2day]

Hi Puffs thanks for your views and I agree with every point you make.

I think you would agree with me though that if the seal, whether it was a washer or an O ring, had quite a degree of "sponginess" or compressibility, the carb would not be likely to move that much to slacken the compression on the seal. In fact it is not likely to move hardly any amount in practice but yes, you still need the initial compression on the seal which makes the rubber O ring ideal but it does need retaining in a cavity of some sort. A rubber washer will also work but I don't know how "squidgy" the silicon one I recommended is so I take back that recommendation.

I agree that obtaining the bevel on the carb is a little tricky but not too difficult to get a decent shape I think. I would use a sheet of coarse wet-and- dry on a surface plate and swirl the end of the carb round and round with a wrist movement holding the carb at the 45 deg (approx) angle to get the bevel on the edge ....it's only soft aluminium and I reckon I could get it done in about 5 mins and it would be pretty even. It doesn't have to be that big, just a couple of millimetres wide

Silicon would work but it is a touch messy and not easily repeatable without lots of cleaning up.

Les

[Puffs]

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