New TS250 Owner

[Prusakowski-Ost]

Thank you for your assistance!

[Puffs]

Well, I just tried that translation method in Firefox, but on a local file (so the HTML in those zip-files): it doesn't work. There are other methods, seehttps://www.makeuseof.com/tag/10-top ... web-pages/ & https://support.google.com/translate/an ... tion&hl=en . In Chrome & spinoff browsers it should be easy, but I don't use Chrome. In Firefox there are translation add-ons too, including DeepL.

[Guesi]

Hello.
The engine has to be taken apart for changing the seals.
When you did that, I would suggest to also change the bearings.They might be rusty due to the long storage....

And the oil draining can be caused by the damaged oil seald. The pressure from the crankcase can be pressed through the damaged seals into the left side of the engine (where the clutch is located (producing an over-pressure there).
This overpressure causes the the oil to drop out of the engine....

[Puffs]

Hi Guesi, happy to continue talking about something else!

Agreed, when the seals leak you'd get pressure on the gearbox. With oil splashed everywhere inside the gearbox, in a running engine, that pressure may push oil out elsewhere, like via the gearshift lever seal, output shaft seal, or near that clutch thingy. It'll leak a bit while running, and then when you park the bike it flows to the lowest point & drips off there, leaving a small puddle below that point. But I'd still think the oil leak is probably not the cause of the OP's running issues, would you agree? Particularly as he mentions the prime issue was that the engine shut down because of losing compression.

The only other cause for a sudden loss of compression I can think of (in addition to head gasket, stuck rings, or excessive piston+bore wear) is that the spark plug was insufficiently torqued & worked itself loose, allowing pressure to escape past it. That would obviously be a very easy fix, but do look carefully at the electrodes (may have molten partly due to lack of cooling).

If it's not something simple as that, I'd still get the barrel off & inspect the piston, bore & conrod bearings. How did the engine sound before it shut down? Did it grind to a halt, did it whistle, ...?

Anyway, if the engine has to come apart, I normally prefer to take the barrel off first, with the engine still in the frame: it makes the engine lighter and easier to handle. And then indeed also replace the main bearings & conrod.

[Guesi]

In the end, it doesn´t matter where the oil comes from, because , as you said, the engine has to be regenerated .

[Puffs]

A little while back Blurredman linked to a video quite relevant for showing how oil (here green) moves through a gearbox, in a running engine (precariously tied onto a rolling road): https://www.youtube.com/watch?v=Bi3dGI5cLu4
This is a Simson S51, which could be viewed as a cousin to the MZ's. Of course the 250's have the clutch on the crankshaft, which rotates even faster.
Anyway, the point is that with pressure on the gearbox, oil can leak from anywhere.

[Prusakowski-Ost]

Sorry for the delay and many thanks for all responses! I took the head off and cleaned everything up....not much carbon after only 1000 miles. Didn't notice any blow-by from the head gasket...cleaned everything up and used the spray copper gasket sealant to ensure a good seal. No love trying to start it....back fired a few times and refused to start. The bike will be a static display for a while until I want to delve into it more.

[Puffs]

I'm sorry, but the idea is to lift the barrel from the engine (after you pull the carb back), then you can see inside & inspect the bore, piston, rings & conrod bearings. A reason for a sudden loss of compression could be something with the rings (although that is quite uncommon in a standard engine). Read in a manual how to take the barrel off, and how to replace it (guiding the rings & piston into the cylinder).

Btw, if that piston has 1000 miles from new, and was cleaned before the pic, I think that is a lot of carbon, which would support crankshaft seal failure as a cause. But the crown does not look like a young piston.

There is an odd indent in the barrel's top seal area, near the left rear stud. No seal problem? Might be a photo issue.

[Prusakowski-Ost]

Yes...jug / barrel came off....rings inspected...no issues. I believe you are correct....crankshaft seal failure as a cause!

[Puffs]

Well, maybe count yourself lucky. Replacing seals (+ main bearings?!) is cheap, compared to a ruined cylinder, which could very well have the same symptoms.

[dave47]

Commiserations. Any idea of the cause?

[Puffs]

Yes. It might be not adapting the main jet to the cold air (which has more oxygen), but I thought I had the jetting pretty much under control, and it hasn't been changed in years. So I had a think about how much I did with that bike. Obviously it's the '92 YZ250 shown elsewhere, which currently has no working distance meter, so it's a guess. And it might be in excess of 30k km, of enduro work alone. Now it's had a new piston (Pro-X) a long while back, but I suspect it's just wear of the Nicasil layer, with the rings wearing through the Nicasil coating. And the moment they hit the aluminium, the friction increases, which causes the temperature to rise, and from thereon the abyss beckons. My top ring was broken.

I think I should have given it a new layer of Nicasil earlier, and another piston. But that would only have avoided the extra costs of welding the scratch up, which isn't all that much. (See for instance https://www.splatshop.co.uk/cylinder-re ... court.html)

Anyway, not an MZ issue, and that's why I had removed the pic. Apologies for the diversion.
In an MZ 2T, with it's cast iron sleeve, you'd just get a re-bore & a new piston - if something like this would happen at all.

[Prusakowski-Ost]

Puffs....collect new seals and come to the U.S. to assist in getting this bike running again......

[Blurredman]

I'm sure he would (I would too) if you paid the air fare...

[Puffs]

I'd love to, no worries! If you arrange a ticket for my wife & me, I'll get seals & some other spares & come & fix it!

In my case the sudden loss of compression was clear: a broken ring had gauged a ditch in the bore above the exhaust port. While the barrel is at the workshop for welding/re-bore/nicasil plating, I had a closer look at the piston:

Key to all doom.jpg (16.82 KiB) Viewed 770 times

This is the piston seen from the intake side. Marked are the areas with a key to lock the rings in their place: it's a pin in the piston, while the rings have a recess, which would fall over it. That would prevent the ring from rotating. The idea is that the biggest hole in the cylinder wall is the exhaust port, and you want the opening of the ring over an area without holes, so that both sides remain supported at all times. In a 4-stroke this is not required, as there are no holes in the cylinder wall.

But in a 2-stroke there are, and it is vital to keep the rings oriented as per design. See that the top pin is very deep into the piston. This will have allowed that ring to rotate - and catch with an open end into the exhaust port!

As evidenced by the shards I recovered from the exhaust:

Rings.jpg (25.69 KiB) Viewed 770 times

On the left the top ring, with the shards somewhat straightened, on the right the 2nd ring, which I managed to get out after some surgery. Looks like most of it is there (rather than in the crankcase), maybe some other small bits left through the exhaust. But also important: you can see one end of the top ring broken off. That should never have been near the exhaust port.

How is that possible? Well, there are some signs of detonation (on the piston, none on the head or cylinder), and that might have contributed, but primarily I think the piston manufacturer (Pro-X) must have drilled the hole for that pin too deep. And over time (after many heating up & cooling down cycles - different materials have different expansion coefficients) the pin went deeper & deeper, till it was deep enough to allow the ring to rotate. My fault might be that I used the piston too long, but never again will I buy a Pro-X. That is actually a Japanese piston, and the alloy seems fine & wear-wise it looks still quite acceptable. But that hole was too deep.

I'm mentioning this on an MZ forum, because this might be relevant for MZ 2-strokes too. The locking pins in MZ pistons are similar (but not identical), and surely it'll be something I'll look for in future.

As for the OP's issue, I am not entirely certain of the cause. Yes, it can be seals (and it probably is), but a dramatic failure of the seals is not so common, IMO. But if you don't have any compression, and if piston & bore are fine, there's no alternative to taking the engine apart and looking for the cause. That probably sounds daunting, if you have no experience with anything like that, but there are good manuals & videos on youtube. Or maybe there's a friend with suitable experience? I expect there is little urgency or stress of time (other than that summer is coming!). Having it done in a workshop will become quite expensive, as it's a lot of man-hours, even though parts are cheap. And there always is the sell as-is option, though that too will cost money.