More MZ mystery.

Wotcha.

Here's an MZ mystery for you. Take one MZ 250/1 - very similar to the one I
bought from TOG. Six volt electrics - mechanical cable drive rev counter -
charge light in speedometer. All in all, a normal, 1980, MZ. No mystery
there, I hear you say. Right. Every now and again, especially when you
pull the bike onto the centre stand, the rev counter stops working, the
charge light comes on ( and the bike doesn't charge and begins to misfire )
and there's a slight mechanical "ticking" from the region of the crankshaft.
Now I can understand the rev counter failing - it's driven direct from the
crank - could be cable / drive unit / rev counter head. . . but why does it
stop charging at the same time ? The dynamo is at the other end of the
crankshaft to the rev counter drive. What's making the ticking noise ?
Stop the engine and restart, and all is fine. There doesn't appear to be
any deifite way to make the fault occur. Anyone got a clue where to start
looking ?

Typical - I get an MZ, and all of a sudden folks turn up thinking I am the
great MZ guru . . . . as if I ain't got enough problems with just the one I
own ;-)

 

[snip]

I had something similar with a KH250. Put it on the centre stand and get
it ticking over. Now push down at the back of the pillion seat and it
cut out. After much mucking about there was a hard to spot wear in the
insulation and if you pressed in the right place you earthed part of the
loom.

 

The speedo worm drive has started to chew into the plastic bush and is
not engaging properly - I suspect the jolt from the side stand causes it
to jump.

The little plastic bushes are cheap and easy to replace, you can pack the
worn part with M6 washers. Take off the speedo drive and check. Be
aware it's used to adjust the clutch so you'll need to reset it. I set
it to 9 - 10mm.

The charge light is probably down to worn dynamo bushes or rusty springs.
Biro ones don't work... Cheap and easy to replace.

Check and clean the fuse box and replace both the fuses even if they look
ok - cutting out and misfires are usually down to the fuse box.

Does it die when you apply the rear brake at idle (voltage drop)?

 

sweller wrote:

Wotcha.

Right - I'll be delving into there tomorrow.

Now I happen to know the brushes are new.

New fuse box too - I fitted the bugger not so long back.

It dies when any load is applied.
The bit that is getting me all confused is the way the charge light only
stays on when the rev counter fails.
You can put a volt meter on the thing and it charges nicely, even on full
headlight.
The moment the rev counter stops - the charge light stays on, and there's a
strange ticking from the engine.
I was thinking along the line of the crank shaft is moving sideways and
disengaging the rev counter drive, and allowing the brushes to move clear of
the commutator on the other end of the shaft.
I will be giving my shaft a good pull tomorrow morning - oooer missus.

 

It's not when the rev counter fails - its when the the bike is put on the
stand (or suffers a jolt)....

Does the rev counter and charging system go down independently of each
other?

As to dieing when brakes are applied or horns parped usually points to a
bad earth or crappy connection (usually at the fusebox). As you know, 6v
systems suffer greater voltage loss from dirty connections.

The ticking could be the rev counter drive rather than the dynamo.

I can't really see this as the commutator is huge, as is the dynamo. The
worm drive on the other end of the crankshaft is industrial spec too (the
little plastic bush isn't, though).

The mechanical regulator can play up and cause problems. It's also
susceptible to jolts if it's old and knackered.

This may help:
http://www.sweller.co.uk/mz/electrics/dyn_wrdia.html

 

sweller wrote:

Wotcha.

So far I ain't seen one without the other.
They either both work, or they both don't.
It just seems a bit too much of a coincidence.

Indeed. This has been re-wired through out - tri rated, 2.5mm.

Rather hard to tell exactly which side it comes from. I didn't have the
time yesterday to take a serious look.

It was more a "clutching at straws" thing. Trying to find something that
the opposite ends of the crank had in common.

I've checked and set the regulator. It does what it's supposed to.

Cheers for that - I printed that out yesterday when I was setting up the
regulator box.

Ah well, I'll have a quick breakfast, then stand in the garage scratching my
head for a while.

 

Daft question, but is everything properly earthed?

When I first bought my Guzzi the electrics were acting strangely, so I
took the clocks off to have a look. At this point I found that the
headlights no longer worked. It was only when I reconnected the speedo
drive that it all sprang back into life.

The dealer had fitted a new battery but had forgotton to reconnect one
of two earth wires to the negative terminal.

Maybe your bike's using the cable as an earth and it's making
intermittent contact? Quick & easy to buzz out.

 

Pip Luscher wrote:

Wotcha.

Indeedy.
All earths ( and more besides ) are good.

Now seeing as I had the time today, I decided to get to the bottom of this
'ere fault. Right, let's get the bike in "fault mode" and take it from
there. I tried, for over an hour, to get the thing to misbehave. No way.
It just would not perform the magic no-tacho-no-charge trick. I took the
thing for a good run round - it ran just as it should. Got back and did
some more wiggling of bits, kicking of bits, pulling at wires, and all the
other things you do to try and annoy the bike enough to make it fail. I
took covers off, checked all terminations, looked at the rev counter and its
associated bits and bobs. I have now reached the conclusion there is a
Gremlin living in my garage, and it likes to feed on MZs. . . . . . . traps
have been set ;-)

Thanks for all the replies - I only wish there was a definite, fixable
problem.