Das MZ Forum für MZ Fahrer

Hello,

Sorry for the title, but I do not know German (maybe one day).

A little help please, I have just re-built my 1987 ETZ250 with new main bearings, and after re-assembly the bike quite easily wants to run in reverse.

I checked the timing, changed points, condensor, taken alternator and rotor off to clear surfaces etc. Still it likes to runbackwards 2/5ths of the time.

I have tried the timing plate in the most severe retard and advance positions and still runs backwards with my test (turn ignition off, then back on before engine stalls, if rev counter doesn't register then running backwards).


A little lost, as this happened in 2017 after I rebuilt the engine back then. I tried everything back then including replacement crankshaft, different cylinder/barrels, all ignition components and many, and in the end out of frustration I bought a contactless ignition- but after a while I gave up, because it still ran backwards with the contactless ignition. The odd thing here is, Out of frustration over the months I decided to try the contact breaker system again, and after timing it in 2017 it never ever ran backwards again until yesterday when I decided to take the rebuilt engine for a test ride... How can this be!? Any pointers...!?

My hair is disapearing!

Hmm, the reason for a two stroke engine running backwards must be a wrong ignition timing, I don't see any other possible reason. Did you check the ignition with a timing disc or a micrometer dial gauge?
If the setting range is too short to get the correct timing, it might be due to an incorrect position of the keyway at the right side crankshaft end. As the key is only a help for correct positioning, you might put the rotor onto the crankshaft without the key, of course in the correct position .

Regards
Andreas

Usually you just dont turn the ignition oft and then turn it back on while the engine is still running
I dont think this is a real Problem.

Ich hatte hier mal dazu was geschrieben, vielleicht hilft´s.
https://pic.mz-forum.com/lothar/ELEKTRI ... waerts.pdf

Flugs mit google übersetzt aber nicht überprüft ...

In the beginning some theory - practical hints at the end.
See here:

lothar hat geschrieben:... Flugs mit google übersetzt aber nicht überprüft ...

Moin Lothar,
mit deepl.com kommen meiner Meinung nach verständlichere Texte heraus als mit google translator.
Herzliche Grüße aus Frankfurt am Main
"Tigerente" Jörn

If you're really in desapair: The "Vape" ignition (complete version!) can't run in reverse direction, because it includes two sensors to detect the right direction.

I think Lothar figured this out correctly 12 years back, I think it's something to do with bouncing of the points near TDC, when the crankshaft rotates backwards. The question is: what to do about it. Because mine never did this. Surely it kicks back a bit, if I kick it wimpy, but it never runs backwards. It must be something in the shape of the cam, or the cam follower, or the spring of the points, or the hinging of the points, something that causes the point to vibrate & make an extra contact near TDC, when running backwards. It might even be play/vibration in the crankshaft's main bearings. An EI should not be susceptible to vibrations, but an EI might still give a spark near TDC.

Replace items one at a time to diagnose...

Ich denke, Lothar hat das vor 12 Jahren richtig herausgefunden, ich denke, es hat etwas mit dem Springen der Punkte in der Nähe des OT zu tun, wenn sich die Kurbelwelle rückwärts dreht. Die Frage ist: was dagegen tun. Denn meiner hat das nie gemacht. Sicherlich schlägt es ein bisschen zurück, wenn ich es schwach trete, aber es läuft nie rückwärts. Es muss etwas in der Form des Nockens oder des Nockenstößels oder der Feder der Punkte oder des Scharniers der Punkte sein, etwas, das den Punkt zum Vibrieren bringt und einen zusätzlichen Kontakt nahe dem oberen Totpunkt herstellt, wenn er rückwärts läuft. Es könnte sogar Spiel/Vibration in den Hauptlagern der Kurbelwelle sein. Ein EZ sollte nicht anfällig für Vibrationen sein, aber ein EZ könnte immer noch einen Funken nahe dem oberen Totpunkt abgeben.

Ersetzen Sie Artikel einzeln, um eine Diagnose zu stellen ...

Tigerente hat geschrieben:

lothar hat geschrieben:... Flugs mit google übersetzt aber nicht überprüft ...

Moin Lothar,
mit deepl.com kommen meiner Meinung nach verständlichere Texte heraus als mit google translator.
Herzliche Grüße aus Frankfurt am Main
"Tigerente" Jörn

Das kann ich als Nicht-Laier bestätigen!

Wobei beide Maschinen gleichmässig überfordert werden können:

IT - Al contrario delle superfici in cemento, la ceramica è più piacevole al tocco di un piede nudo.
DE - Im Gegensatz zu Betonoberflächen ist Keramik angenehmer zu berühren als ein nackter Fuß. (courtesy of DeepL)

Tigerente hat geschrieben:

lothar hat geschrieben:... Flugs mit google übersetzt aber nicht überprüft ...

Moin Lothar,
mit deepl.com kommen meiner Meinung nach verständlichere Texte heraus als mit google translator.
Herzliche Grüße aus Frankfurt am Main
"Tigerente" Jörn

Naja, soll ja keine Shakespeare-like-Übersetzung sein, sondern nur der schnellen Hilfe dienen ...

Gruß
Lothar

PS:
Übrigens, wer Lust hat, die MZ-Elektrik mal ins Englische zu übersetzen, herzlich gern.
Vielleicht gibt es auch (englisch-muttersprachige) Schüler oder Studenten, die auf sowas
Lust haben, oder sogar MZ-Fahrer ... Geld winkt keins, nur Ruhm ...

Wenn ein Motor Rückwärts läuft, muss es nicht die Zündung sein. Ich hatte das Problem beim Berliner und da war es der Vergaser. Viel zu Fett und dadurch das Standgas sehr langsam. When an engine runs backwards, it doesn't have to be the ignition. I had the problem with the Berliner and there it was the carburetor. Much too fat and thus the idle very slow.

Lg. Mario

...too rich ...

max ~:)

Hi,

Thank you all for replies.

I did some more last night, changed points, checked cam (a little scored but it can't have changed so dramatically in 1 month of non use ). I took the washer off from between cam and rotor to see if the heel of the contact breaker might ride upon a different surface. It changed nothing. I changed the rotor itself too and timed but still the same.

0.4mm gap also gave backwards running. The only thing which didn't give backwards running is to have the gap at 1mm, but that's not a fix...

Changed contact breaker adjuster plate and it all timed up exactly as before.

I'm a little lost indeed. I suppose the only thing I can do for now is steal the electronic ignition from my 251.. But that's not a proper resolution. I would like contact breakers, ideally. It would however be interesting to see if the 251 runs backwards with the ignition system of the 250, and vice versa. It may not be worth it though if the EI I have on my 251, allows the system to run backwards too.. I can do a simple test there to determine whether there is any point in swapping the systems between bikes.

The contactless ignition I have currently on my 251 is this:


I timed with a rod in the spark plug hole at roughly 2.5mm BTDC, and used a light bulb to accurately gauge contact breaker opening.

Während sicherlich der Vergaser, das Kraftstoff-/Luftverhältnis und das Öl-/Kraftstoffverhältnis den Lauf beeinflussen und es auf diese Weise wahrscheinlicher (oder weniger) machen können, dass der Motor einmal zurückschlägt, ist das Verdichtungsverhältnis in einem ETZ viel zu niedrig zur Selbstzündung (wie bei einem Diesel, der typisch einen CR von ~1:20 hat). Um weiter rückwärts zu laufen, braucht es einen Zündfunken nahe OT, wie auch Lothar in 2010 erklärte.

Das Rätsel ist: Woher kommt dieser Funke? Ein herkömmliches Batterie/Punkte/Spulen-System gibt keinen Funken, wenn Strom beginnt auf der Primärseite der Spule zu fließen, sondern nur dann, wenn er plötzlich aufhört zu fließen. Also nur wenn die Punkten öffnen , bekommen Sie einen Funken, nicht wenn sie schließen. Wenn der Motor rückwärts läuft, erhalten Sie einen Funken in der Nähe des UT, nicht in der Nähe des OT. Im Idealfall.
Aber wenn die Punkten beim Schließen etwas hüpfen oder klappern und in der Nähe des OT, bei rückwärts laufen, könnte das ausreichen, um einen Funken zu erzeugen? Offenbar; es scheint die plausibelste Erklärung zu sein.

Wenn Sie die Hubhöhe auf 1 mm eingestellt haben, lief der Motor nicht rückwärts. Ich nehme an, das Timing war noch Standard. Bei dieser Hubhöhe beginnen sich die Punkte an einer anderen Stelle auf der Nocke zu heben (und auch zu schließen), höchstwahrscheinlich dort, wo die Anstiegsrate geringer ist. Wenn das Anheben oder Schließen (je nachdem, in welche Richtung der Motor läuft) sanfter und weniger abrupt erfolgt, sollten die Punkte beim Rückwärtslaufen in der Nähe des oberen Totpunkts weniger zum hüpfen oder klappern neigen. Und ohne das - kein Funke und kein Laufen. Das würde also Sinn machen.

Warum kannst du nicht mit einer höheren Hubhöhe fahren? Solange du es noch richtig timen kannst.

+++++++++++
While surely the carburettor, fuel/air ratio & oil/fuel ratio will influence how it runs, and can in that way make it more (or less) likely that the engine kicks back once, the compression ratio in an ETZ is way too low for self ignition (as in a Diesel, which typically has a CR~1:20). To keep on running backwards, it needs a spark near TDC, as also explained by Lothar in 2010.

The enigma is: where does that spark come from? A conventional battery/points/coil system will not give a spark when current starts to flow the primary side of the coil, only when it suddenly stops flowing. So only when the points open you get a spark, not when they close. So when the engine runs backwards, you get a spark near BDC, not near TDC. Ideally.
But if there is some bouncing of clattering of the points when they close, running backwards near TDC, that might be enough to generate a spark? Apparently; it seems the most plausible explanation.

When you set the lift height to 1mm, the engine did not run backwards. I presume the timing was still standard. With that lift height, the points start to lift (and also close) at a different location on the cam, quite likely where the rate-of-rise is less. When the lifting or closing (depending on which way the engine runs) happening smoother & less abrupt, there should be less tendency for the points to bounce or chatter when running backwards, near TDC. And absent of that - no spark, and no running. So that would make sense.

Why cant you run with a higher lift height? As long as you can still get it timed right.

Okay,

So I have found something interesting. I have taken some spare parts from my brother's ETZ250, in order to implement the points on both my ETZ250 and ETZ251, and I find, even with completely different ignition components that both bikes have a maximum contact breaker gap size @ TDC, and do not prolong to be open to close (from opening point) for roughly half the rotation of the crankshaft. Infact the contact breakers on both bikes start to rise (as timed by me) at 2.5mm BTDC, and then as they just swipe past TDC they are already closing and are closed at about 2.5-3mm AFTER TDC. Therefore I think this is where the issue is. Total points opening is no more than 50 degrees of rotation!

What is strange is that this occurance is on both 251 and 250. Both different cams, crankshafts, contact breakers etc obviously.

But: The way around it? I suppose the only thing I can think about if I want to retain points is to see whether I can open up the gap of the points considerably to see whether I can prolong the amount of time before they close.... But then whether the timing could be adjustable to compensate for that I really am not sure.

Another possible thought is that I could implement some sort of shim between the cam and the rotor opposite the highest lifting point of the cam in order to influence a longer opening time.

What are people's thoughts?

Hello,
this is unfortunately correctly observed, but wrongly thought.
The only thing that is interesting is the correct point at which the interrupter contacts open in relation to top dead center. - In the correct running direction of the motor!
The period of the previous closing of the contacts means the energy charging time of the ignition coil.
The components used in the ignition system must match the engine, otherwise such confusing results can be observed.
Please check with a suitable spare parts list.
best regards Uwe

Hi,

Yes, so the problem is the points are closing far too soon, and this obviously creates the problem of running backwards, but equally so means the coil is charged too much and causes issues there too.

But my solution wasn't the best implemented and didn't seem to work.


Does this sound like the crankshaft is out of true?

The cheapest and quickest solution is electronic ignition but I want to keep the bike standard.

Hello,

yes, without the right components, unfortunately, no success can be expected.
I refer again to a spare parts list for the motorcycle. Should be able to find it on the internet.
At Güsi-Motorradteile there is also an electronic ignition on offer, with an electronic igniter instead of an interrupter.
Of course, the crankshaft must not have an impermissible height difference, you must not be able to wobble it.
Otherwise you can't adjust anything properly.
The charging time of the ignition coil does not matter.
regards Uwe

Well, agreed, even with a quite short charge time you can get a spark when you break the current in the primary side of the ignition coil. But therein lies the rub: you only get a spark when you break the current, not when the current starts. So to get that spark, which it needs to run backwards, the points need to lift near TDC when running backwards.
The OP's short opening time causes the points to close 2.5-3.0mm aTDC when turning in the normal direction, so when running backwards they open 2.5-3.0mm bTDC. Which is a fine timing for running! So it can run backwards, enigma solved.

Now why it has such a short opening time is another question. If the cam is standard & fitted properly, it has to be because the end of the rotor, on which the cam is fitted, is out of true. This is possible if the rotor is fitted poorly, or if the shaft (=the end of the crankshaft) is out of true. For a 250 single, crankshaft imbalance is typically a max of 0.02 - 0.03mm, which you would also see at the end of the shaft, when the lot is fitted in the main bearings. Now the end of the rotor is further sideways, and that will amplify that number (by the ratio of distances to the bearings). Say typically 0.05mm max at the end of the rotor? Which you can measure at the end of the collector.For a better measurement you'd have to fit the dial (which was not zeroed in the pic) to the engine, rather than in a stand as I did here (easier).
Measure not only the run-out, but also the direction in which it deviates wrt the location of the points' cam follower - that should explain the observed lift profile.

(Without a dial indicator, make a solidly fixed pin close to the same place & measure with feelers.)

Okay guys,

It seems like my crankshaft is not balanced and this is my problem. 0.55mm runout. I will be rebuilding the engine!

I have rebuilt engine with a replacement crankshaft. Measured the run out at 0.02. Now no more problems. Have yet to fully utilise the bike but have been on a few small 10 mile rides for some local erands.

Thank you for reply