How does (and should) electronic ignition advance work?

[LexH]

Hi All,

First post on bevelheads. I've searched the forums for this topic but couldn't find answers. Apologies if this has already been discussed.

I have a relatively newly acquired '72 450 Desmo Silver Shotgun that I'm currently trying to sort. It has a Ducati Elettrotecnica electronic ignition which I'm told was fitted as standard.

My problem is trying to get the ignition timing right. What I'd like to know is how does the advance work with these systems, and more to the point, how is it supposed to work? I've read posts about the advance increasing with engine speed and this being an inherent property of the magnet/pickup design. However, when I observe timing versus rpm with a timing light I see no change to ignition timing at all, from idle to high rpm.

Currently the spark occurs at close to TDC, which is fine for idle, but when the engine is revved the timing doesn't appear to advance at all.
Needless to say the engine feels a bit sluggish on the road, and I've also noticed to my alarm that the top of the header pipe will glow dull red if the engine is revved for a few minutes. Not good!

What should I expect to see with the timing light? Increasing advance with rpm? How much? Do I have a faulty pickup or transducer unit?



Cheers,
Lex

[machten]

Hi Lex,

The CDI system should advance progressively up to a max of 36-38 degrees BTDC at 2500 to 3000 RPM. I'd carefully check all ignition wiring connections for poor connection or shorts, particularly those wires going into the transducer. If that is all in order, you may have a transducer problem.

My CDI bike is in the UK, so I can't check it, but I notice the specs also specify the pickup air gap as 0.2 - 0.3mm. I know having the air gap wrong on the Bosch based twin systems can cause all sorts of funky things to fail with ignition advance, but I've never had to mess with the singles CDI systems other than setting the timing. Might be worth checking anyway.

Kev

[LexH]

Thanks Kev, I thought the advance should be something like that.

Do you know if the advance is controlled electronically inside the transducer or is it something to do with the change in pulse shape from the pickup as the revs increase?

I checked the pickup output with an oscilloscope tonight, it produces a single positive-going pulse each power stroke which seems normal. The transducer wiring seems right but I'll check it again carefully. If I find anything wrong I'll blame George!!

I've seen transducers from 860s etc on ebay, they look physically identical to the one on my 450, but if the advance is controlled inside the unit then maybe tranducers from a twin might have a different advance?? Any thoughts, would they be interchangeable?

As far as I can see the air gap isn't adjustable on the ducati elettrotecnica setup.

Cheers,
Lex

[machten]

Hi Lex,

I'll preface these comments with the disclaimer that I'm no expert on these particular CDI systems, but assuming they work the same as others I am familiar with, all the advance timing smarts sit in the transducer. What you're seeing from the pickup is what I'd expect, just a pulse.

I'm not familiar with the 860 setup at all having never owned one or even worked on one. You could try chasing it down through the part numbers, or I'm sure others on this forum will chime in.

Compatible aftermarket transducer units are available if need be, in any case. My 250 is a points job, but there are a few guys around with CDI models that I'd think would lend you a transducer to test it prior to buying one if that was the issue (106 Euros, so not cheap).

As far as I can see the air gap isn't adjustable on the ducati elettrotecnica setup.

Yes, I didn't recall seeing such a facility on my bike, but I've only set the timing on that once and they did mention it in the Haynes manual I have. Maybe it was talking about the Motoplat system that was also used.

If I find anything wrong I'll blame George!!

Good luck with that one!!! I'll lay you London to a brick he quotes the "driveway warranty". Once it's off the driveway, the warranty is expired! (And to be fair...it did leave the driveway, what, 15 years ago? Ahhh, what the hell, I still intend blaming him for anything that happens on the Darmah engine he rebuilt for me in 1981, but I'm still waiting.. )

Kev

[Craig in France]

Hi Lex,

Really just to confirm what Kev said:

1. I'm pretty sure the running advance is done by the transducer box. The pick-up is just a pulse generator. You can move its backplate to adjust the static advance, but that's all.

So, if the strobe is showing no advance, I'd suspect the transducer.

2. The 860 transducers have a different Ducati Elettronica code number from the one fitted to the singles. So I suspect/have always assumed that they're not interchangeable.
EDIT: Sorry, I think this is WRONG. See post below.

3. Repairing transducers is, by all accounts, not worth contemplating, not least 'cos de-potting the epoxy is a filthy and time-consuming task .

4. New transducers for the singles are available from the Amici dello Scrambler. But at a price .
http://www.ducatiscrambler.com/2006/elettrici.htm

So perhaps try Gowie* first - you never know.

* Ah - or Phil at Road and Race, of course. http://www.roadandrace.com.au/electrical_repairs.htm

Ciao

Craig

[murray450]

Hi Lex

I've got some spare transducers (and also a couple of pickups) here in Melb, all good cond or new. Never realised that twin & single transducers may be different. Anyone know where I can find the part numbers of each so I can see what I got?? Anyhow, if you need I'm happy to post one over to try... Cheers David

[Craig in France]

Never realised that twin & single transducers may be different. Anyone know where I can find the part numbers of each so I can see what I got??

Hi David (and Lex),

Mea culpa - on closer inspection, I now think that what I wrote above was a load of tosh! Sorry!

The Ducati part number for the transducers for both the singles and the 860s is, indeed, the same, namely:
0616.46 303.

And both have stamped on them, 32.39.04.

So I guess they are interchangable.

Ciao

Craig

(And there's me been thinking all this time they were different. Tsk!)

[Jon Pegler]

The Ducati Electronica transducers were also fitted to Morinis and are interchangable with the ones used on Ducatis.
They differ only in that they have an extra terminal that is used to power an electronic tachometer.
I have used them on Ducati singles with no problems.

[MorDuGuz]

Hi Ducatisti !
On the Dutch Morini Club site (www.motomoriniclub.nl), you can find valuable informations on the electronic pick-up and transducer (section technical and hence electronics).
There are complete papers (in german), written by Marcus Heilig and especially by Franz Grabowski.
It seems the delay is 47 ms at 1000 rev/min and 4.7 ms (in advance of course) at 5000 rev/min (30° ?).

Have you checked the main jet size? I suppose a bad fuel/air mixture or pressure has some effect on the spark timing, but probably not to this extent.
You seem experiment the reverse of self-firing!
Jean-Louis

[Harvey.]

Thanks Kev, I thought the advance should be something like that.

Do you know if the advance is controlled electronically inside the transducer or is it something to do with the change in pulse shape from the pickup as the revs increase?

I checked the pickup output with an oscilloscope tonight, it produces a single positive-going pulse each power stroke which seems normal. The transducer wiring seems right but I'll check it again carefully. If I find anything wrong I'll blame George!!

I've seen transducers from 860s etc on ebay, they look physically identical to the one on my 450, but if the advance is controlled inside the unit then maybe tranducers from a twin might have a different advance?? Any thoughts, would they be interchangeable?

As far as I can see the air gap isn't adjustable on the ducati elettrotecnica setup.

Cheers,
Lex

Giday Lex.
I think it is unusual for the 72 Special to have the later electronic ignition. My 72 one had points, but they were not all built out of the same parts box.

The advance on the later electronic was built into the pick-up, and depended on the speed of the rotor.
The gap between the stator and the rotor is graded, with the leading edge of the stator wider than the trailing edge. At low speed it needs the narrow gap, of the trailing edge, to generate a good pulse (retarded).

As the speed increases the pulse is generated further along the leading edge of the stator, because of the higher rate of flux change, till at 2500-3000 rpm, it is the leading edge that will generate a strong enough pulse to trigger the CDI unit.
The same arrangement was used on the later Bosch advance.
The CDI unit is the same as the unit that was fitted to the 76 750SS.

[Steve Foster]

G'day Harvey

The gap between the stator and the rotor is graded ... As the speed increases the pulse is generated further along the leading edge of the stator, because of the higher rate of flux change, till at 2500-3000 rpm, it is the leading edge that will generate a strong enough pulse to trigger the CDI unit.

Thanks - nice explanation of the principle.
Does anyone have a picture to illustrate how the rotor-stator gap changes depending on their relative positions? That would really clear things up.
Cheers
Steve

[Harvey.]

Giday Steve.
Not easy to show as there is nothing to see. There are two ways of doing it, the Bosch on the later 78-80s CDI has three defined peaks that are different distances away from the pick-up, and can be seen.

The 450 pick-up rotor uses the amount of pole that overlaps with the stator pick-up pole. There is no defined point of maximum magnetic flux flow. It depends on how much of the rotor pole is overlapping the pick-up, that is flowing magnetic flux, and how fast the lines of force are being cut (speed of rotation).
At low speed it needs more of the rotor pole passing magnetic flux to generate a voltage. As the speed of rotation increases, because of the speed of flux change, less of the pole needs to be overlapped, to generate the voltage. So the spark is advanced gradually as the speed increases.

You can see that there is no defined point of triggering, just two long sections of poles on the rotor, that pass the magnetic flux across to the stator.