ETZ 250 - 251 diffrences

[Puffs]

@ Dave47: yes, if both are EM250 barrels, they should only differ in the depth of the exhaust port. Maybe your '89 barrel is actually an EM251 barrel? At least timing-wise it appears to be.

For the record: mine was first registered 14/08/'89 (in the UK), and Haynes say the 251 was introduced in 07/'89. I haven't checked the timings with Guesi's picture myself, but it should be those of the 29-42.025, where they took 2mm off the base (note the pic says 2mm, not 1mm). But I cannot find that number, at least not at the indicated place. If I remember correctly, there is a number stamped somewhere, but I'm not going to take it apart just to check. Does your's have numbers? For the EM250 it should be 29-42.017, for the 251 it should be 29-42.025.

My measurements were done after the barrel had been re-bored, to 70.50mm, that may make the exhaust a bit wider (and the base deeper). And it's also quite possible that I widened the exhaust a little, I cannot remember (bought the bike 21 years ago), but it is something I might have done.

@ Alexxx: if you keep the exhaust (& overflows) open for a shorter while, as you do when the barrel is dropped by 2 or 1 mm, you pump less gasses into the exhaust, thus reduce the average back-pressure in the exhaust a bit. So you can get by with a slightly lower muffler volume, hence a shorter exhaust. But the result is reducing power output, and lower the RPM peak. It's going towards old-school timing. And it conflicts with what they did to the inlet. But most importantly, an exhaust like the standard one is non-reflecting, hence timing of the exhaust system does not come into play. It's only back pressure, and back-pressure is not the same as a reflected pressure pulse, as you get from an expansion exhaust - that IS timed, and can push back fresh mixture into the combustion chamber just before the port closes, thereby giving the effect of pressure charging (like you get from a turbo or other inlet compressor).

That's the long answer, the short answer is: I don't think a longer exhaust will make much difference, but it is more awkward, as it sticks out more.

[Blurredman]

As an aside, I wonder if Guesi has blueprints in regards to the 15 vs 21 hp difference found on different ETZ 250's? The lower hp item being available for certain exports apparently.

[dave47]

@ Puffs, If your number is not on the front, it will be on the LHS of the "neck", in the middle, but it is unlikely to tell you much. It will almost certainly say 29-42.000. On mine there is another number, 710, to the right of that. It may be coincidence that this is 017 backwards.
I say that my barrel has the appearance of a 250 not 251, because it has no frontal number, and what numbers it has are the same as the 1985 barrel.
On the RHS of the neck are two small numbers which almost certainly indicate the month and year of manufacture, eg 89 and 8. They might be upside down.

[Guesi]

As an aside, I wonder if Guesi has blueprints in regards to the 15 vs 21 hp difference found on different ETZ 250's? The lower hp item being available for certain exports apparently.

Hello

In Western germany the hp´s were reduced to 13 kW because of insurance reasons, because we had insurance classes up to 17 HP (13 kW) and 27 HP (20 kW).
So the original 21 hp engine went to the more expensive 27 hp class.
It was reduced by a "longer"downpipe marked with the MZ sign and 13kW.

In the GDR these downpipes were used in the police and military models, because these pipes gave the engine more torque in lower revs.

[Puffs]

Ta Guesi, interesting. For reasons not too dissimilar there is currently still a cut-off at 11kW (15HP) for 125cc bikes in Belgium, France & Spain (and Italy? more?).

Dave47, ta, but no.
I have the following markings cast in the neck of my barrel (with North being forward & West being left):
NW: Si (that's what it looks like)
SW: 710
SE: 1794
East (in the middle, on the overflow port): 68 (with a circle around it), and 6 (+ also sort of circle).
NE: an unintelligible wiggle

At the front (N) there is nothing, and at the back I have a manifold.

Yes, the markings on the RH overflow port (East) could be read upside-down as '89' and '9', possibly suggesting this barrel was cast the month after the bike was registered. That is possible if it's not the original barrel. I bought the bike 9 years old with about 8k miles on the clock, and I always thought it was pretty much original, but it might have received a new barrel. Or those markings are not the date of casting. Or they should not be read upside-down either: the '1794' should clearly be read the normal way.

Btw, if your barrel was cast in August '89, it should be an EM251, and that would explain why the port timings are similar to mine.

I do remember there is a rather longer number stamped at the top of the barrel, below the head gasket, and that might be the elusive 29-42.025; but I'm not going to take the head off just to look.

[dave47]

@ Puffs,
The number will be where I said, but may be obscured to a greater or lesser extent by the LH cover which for some reason is higher than the crankcase. It is mostly visible on my early ETZ, ( broken fin makes it even more visible!) but only the top part of the final .000 can be seen on the later bike. It tells you nothing anyway.

All the markings are the same on both of my barrels and on yours, except the date marks. I think it very likely that this is what they are, because apart from anything else they are in circular panels, which suggests they are changeable. This would indicate that your bike, like mine, has a replacement engine/barrel, but whether from a late 250 or early 251 is anybody's guess. I suspect that MZ may have used the same for both for at least part of the period 88/89.

[Puffs]

Yes you are right, I can just about see something that looks like the very top of the '000' at the end. But I don't have a broken fin, so to see it properly I'd have to take the barrel off. (The angle & view of your 2nd photo is only possible without that lowermost fin.)

LH barrel neck_2.jpg (21.83 KiB) Viewed 13 times

A pic of mine. Anyway, then that number is the same for all barrels, hence indeed useless for detailed identification.

On the numbers in circles on the RH side: I'm not yet convinced those should be read upside-down, and/or that those are casting dates. It's possible, but I'd be a little surprised if mine was not the original barrel. How's your '85 barrel marked there? Does anybody else have a view/ info on this?

It's not really 'anybody's guess' if they are 250 or 251 barrels; the numbers & measurements given in the documents Guesi's provided can serve as guidance, particularly Maß A. That should be easy to measure accurately, but also the port timings are an indication. Next time I have the barrel off, I'll have a look.

FWIW, it looks like the 'Si' & the '710' on your first photo are a bit lower than on your 2nd, which would be consistent with Maß A being (2mm?) less.

[dave47]

I think you are right , it has been dropped by about 2mm, (not 1.2!), which makes it the 251. The data sheet must be wrong about the position of the number. The footnote must only apply to the later 251/301s.
The "datemarks" on my earlier bike read "E" and "85", which I take to mean May 1985. It is consistent with being on a D reg bike (UK), but somebody must know for sure.

[Puffs]

It turns out that at the behest of my neighbour Dr. Alzheimer I had made some pics when it had it's latest rebore:

ETZ251 barrel, ID numbers neck.jpg (23.01 KiB) Viewed 13 times

ETZ251 barrel, top side.jpg (32.5 KiB) Viewed 13 times

So indeed, '29.42.000' and '710', while the stamped '0' @ the top tells me very little.

[Guesi]

29-42.000 is the spare part number

the "0" is only important if the cylinder is new and not rebored.

It says that the cylinder is bored exactly at 69,00 mm, so the piston must be 68,95 mm. Because the clearance on ETZ engines is 0,05 mm.

If the cylinder is marked with 1 then the cylinder is bored 69,01 mm, so you need a piston with 68,96 mm .
If it is marked with 2 then the cylinder is bored 69,02 mm so you need a piston with 68,97 mm.

When the cylinder is rebored, the "0" does not have any importance...

[Puffs]

OK, but according to that MZ document it should have been 29-42.025 for the 251, or 29-42.017 for the 250, and placed at the front.

Anyway, that document appears to be somewhat inaccurate on those details, but what I have learned from this conversation, is that there are differences between the 2 barrels.

Ta also for the info on the '0', makes sense! Other brands use markings like 'A', 'B', 'C', 'D'.

[Puffs]

I've had the barrel of my '89 251 off, and referring to viewtopic.php?f=5&t=9624&p=49338&sid=2aecf9a7e375fbcba527282589100173#p49338 , indeed:

Maß A = 13mm.

Maß A = 12.9mm.jpg (30.96 KiB) Viewed 13 times

See in Guesi's figure that the in '89 new 251 has a (much) longer intake opening, combined with shorter overflow & exhaust timings. It looks like they just chopped off 2mm from the base of the barrel, and added that to the top, while keeping the casting mould. This may explain differences in character between bikes, and why mine benefited so much from reeds. After just 1 year they went to a happy median Maß A = 14mm, possibly realising they didn't really like the new character?

[dave47]

You have confirmed what we thought was the case. Did you have any other reason to remove the barrel, or was it just to service it before changing to premix?

[Puffs]

Now that the nice weather is over, I wanted to have a look at it's internals: barrel, piston, bearings. Before trying another carb on it.

'we'???

[dave47]

'we'???

I thought that we??? had decided in May that we both had early 251 engines with likely A=13mm, and I also had an ETZ 250 engine with A=15mm, as per Guesi's data sheet. You have confirmed one of them, but I will not be measuring mine in the near future.