Sodding MZ

I must have tried every trick in the book. Prime, reserve, not on at all.
The only way it would fire was with no choke, twist the throttle like
buggery, close the throttle and hit the button. It would eventually fire on
one cylinder, then, after more of the same, two cylinders would wake up,
and, if the battery was still up to it, all four. I would then pull the
choke and let it run for a while. Bump starting made no difference at all.
As long as the bike was in regular use, it would fire up by merely pulling
the choke out and hitting the button. . . . . . but if you left it for two
weeks. Argh.

 

In communiqué <1iyl9ey.jiqzc81dftpf9N%>,

On a Morini you can choose which pot. Using both starter jets on all but
the coldest of days results in too rude an awakening.


--
+----------------------------------------------------------------+
| Pete Fisher at Home: |
| Voxan Roadster Gilera Nordwest * 2 Yamaha WR250Z |
| Gilera GFR * 2 Moto Morini 2C/375 Morini 350 "Forgotten Error" |
+----------------------------------------------------------------+

 

When I had an MZ I found that the user handbook and the Haynes manual gave 2
different advance settings - neither of which worked at all well, setting it
halfway between the two worked much better.

 

In future - £40 ultrasonic cleaner from Maplin, caustic soda and hot water,
throw the sooted up plugs in and let them fizz till the oil floats to the
surface.

 

ian field wrote:

Wotcha.

Would this work if we scaled it up a tad ?

Y'know - it might just work.

 

it doesn't. But on an old engine, not all cylinders/pistons/valves etc.
will be exactly the same any more, and chances are one will be the "best" of
the bunch.

I merely relate that I've known several engines where by accident I've
discovered that having one particular cylinder not firing (dud plug, lead
accidentally off, etc.) the engine flatly refused to start on the starter,
and if bumped at high enough speed, would start and run on 3. re-enabling
the 1 missing pot solved the problem. note that all these are old-fashioned
coil-and-contact, not electronic, ignition. On such systems, voltage drop
when running the starter is more critical.

If the remaining pots were prepared to start on the starter, then it should
fire up and run on 3.

Now I think on it, I had another one with a head gasket on the way out,
which showed similar results, wouldn't fire and pick up unless you pulled
the plug which had coolant on it and dried it out. That one did actually
fire on other cylinders, but didn't pick up. Once running, it'd run on all
4. That's perhaps atypical. The other engines weren't thus afflicted, they
just didn't start.


OK. you say it's nonsense and demand an explanation. That's my best guess.
You explain to ME how the engine failed to start on one of the 3 remaining
pots, and then run on 3.

 

What did I say about the plugs - to your disbelieving snorts?

 

Oh yes. Abject apoligies. One duff plug I caould accept, but two?

 

TOG@Toil wrote:

Wotcha.

Hence the "Sherlock Holmes" approach.
I've been caught out by the "impossible" before now.

 

Years ago I worked for a firm that made industrial ultrasonic cleaners, they
often left units up to 1500W on soak test overnight, naturally I decided it
would be a good idea to string my collection of fouled plugs on a length of
wire and leave them dangling in the test tank overnight. They cleaned up
with perfect white insulators but on some the packed in seal was damaged and
blew out under combustion pressure leaving the insulator loose to slide up
and down in the metal body.

There has been no such problem with the Maplin unit.

 

I freely admit that I've not tested the theory. However, on 2 different
motors for certain, having 1 cylinder not firing made them impossible to
start. Whether this would apply to any cylinder, I don't know - Having
fixed the problem, I didn't look any further.

Taking a 4-cylinder engine, firing 1-4-3-2, if for example 1 was missing but
4-3-2 were OK, I'd have though that the momentum created by the other 3
firing would carry it past 1 with enough speed to fire 4 again - after all,
most engines will (just) idle on 3 - and when you're trying to start it,
you're not generally using idle mixture level.

 

Indeed, and the voltage drop theory just doesn't hack it - either
there's enough juice for a viable spark or there isn't. There
s no difference betwee a #1 spark and a #3 spark.

http://xkcd.com/552/

You've observed, in a couple of Shite Old Engines, that one cylinder
seems to fire up on a cold start more easily than the others. Plug,
valve clearances (and timings due to uneve cam wear), choke/mixture
settings etc. etc. will all have an effect, and it's quite
conceivable, indeed completely normal, that in a SOE they won't all be
contributing equally.

How you get from that observation to:

I've no idea.

Sure, most engines should be able to run on three. What's your point?

 

hehe.

I did say that I'd not done any further testing to prove or disprove my
theory.

Maybe one day I'll have time and inclination to pursue it.

The voltage drop thing is perfectly valid though: the coil/contact woks in a
similar way to a transformer: you put 12V in and get maybe 10,000V out. If
you put less in, you get less out. Which is why a lot of things had a 6v or
9V coil and a ballast resistor which was switched out of circuit when
cranking.

The point about the SOEngines is exactly what I said, and you repeated: the
4 (or however many) sets of components are not all equal, due to
differential wear etc., and also due to statistical variation in manufacture
of thing like plugs, even if you fit a new set.

If the voltage drop when cranking lowers the plug electrode voltage to a
point where it's in the critical zone where it will spark/not spark
depending on minor variations such as these (and indeed mixture) then you
can easily get one pot that will fire more reliably than the others.

Modern electronic systems generate higher voltages than the older systems,
which means they're less prone to this issue. Also, injection systems
usually get the fuelling more accurate, meaning it's more likely to fire.

 

Ah. I thought you were implying that only one cylinder fired *by
design*! Hence my somewhat derogatory responce.

But yes, I guess that an engine can where to the point that it's
always the one cylinder that's in best condition (spark plug, coil,
compression, etc) that catches first on starting. But what that
really means is that the rest of your engine is knackered.

 

Not necessarily. The cylinder that starts first could be the one out of
spec, for instance running rich due to a worn jet or a partial blockage.

 

Thus spake Austin Shackles () unto the assembled multitudes:

ITYF that only applies to Chinese motorcycles... ;-)

Sorry, couldn't resist.

 

Champ wrote:

Wotcha.

Now you can see why I like singles - I always build 'em with the one pot
that starts ;-)

 

Did you fit the pot that didn't start to the MZ before you flogged it back
to TOG?

 

platypus wrote:

Wotcha.

.. . . . and I would have got away with it if it wasn't for you meddling
kids.

Speaking of MZs - I went over to pick up the "few bits and pieces" that came
with my recently acquired ETZ. I had to make two trips in the work's van,
the guy I picked 'em up off followed me with his trailer full of bits, and
the boot of his car rather packed too. I dread to think what I'll be
sorting through in the morning - I counted at least twelve wheels going in
to my garage.

 

Twelve wheels? You could go MZ endurance racing with that lot.