Which Coil for CDI Ignition?

[Stratos23]

Thanks Hans. There's some good stuff in there. I haven't digested it all (!), but already it's clarified - for me, at least - how the voltage in the secondary winding in points (Kettering) and CDI systems is created.

I think there's possibly also some relevant stuff for Greg. I note the stuff on the differences between a CDI coil and a points (Kettering) coil. Might explain his problem, if Electrex have supplied him with the wrong type ...?

As always Hans, you're an encylopaedia of wonderfully relevant information! And thanks Craig for the clarification on my points above - of course you're right about the ignition advancing as the revs increase. I'm still however curious about the theory of varying crank speed in big singles. I did hear some time ago that certain aftermarket CDI manufacturers were reluctant to recommend their systems to engines over 350cc.

Anyway, today I persevered with the set-up using the coil supplied (until I replace it with a Dyna 0.6ohm I now have on order). I did manage to start the bike a few days ago - and once running, it does all the right things and seems eager to rev. But today, it back-fired viciously on every occasion - and the fourth attempt was so violent, that the impact of the lever kicking back was enough to throw the circlip off the fold out 'peg' - which turned into a deadly projectile, hitting me in the mouth and causing quite a bit of damage (to me, not the peg!!). I'm very lucky it wasn't my eye. Since installing the Electrex CDI, it's always had a tendency to back-fire like this. I've tried retarding the ignition incrementally (which made no difference). I've checked and checked the ignition timing several times over. I've carefully observed it's advance under strobe and it's doing what it's meant to.

This is a basic description of the Electrex system:
- It's a 12 pole stator which has 6 charging cycles per rev (according to an email from Electrex)
- It fires on both ignition and exhaust stroke
- It advances by 25º at 4000 rpm (they haven't told me exactly what the curve is - how progressive this is).
- With their recommended timing marks, the spark is discharging at 3º BTDC at kick-over and advancing to 28ºBTDC above 4000rpm

I'm not even going to try and attempt to start the bike again until I have a replacement coil, which hopefully might make a difference. I feel I've been going round in circles and all I have to show for it today is a mangled face! And ironically, each day I'm riding a '74 250 Desmo (with original Ducati electronic ignition) which starts effortlessly - first kick every time!

I guess the burning question is HAS ANYONE ELSE OUT THERE HAD ANY EXPERIENCE WITH AN ELECTREX CDI on a Ducati single?? - particularly on a 450? Electrex aren't even replying to my emails now and I'm wondering whether I've made a bad call investing in it (they're not cheap).

Thoughts?

[Jordan]

Just to try it if you haven't already: Change over the two wires connected to the inductive pickup.
Depending on design, there can be different behaviour, relating to the shape of the pickup waveform.
Sometimes, if the waveform is not symmetric, advance behaviour can be quite different if the wires are changed about.
It's a long shot, but you never know. Easy to try.

[Ventodue]

You are having a right merry time with this bike, Greg!

- With their recommended timing marks, the spark is discharging at 3º BTDC at kick-over

Err ... static timing for a 450 is 0°. Enuf to make a difference? Dunno ... I have read of a bike kicking back with just 1½ degree of advance, so maybe.

(You are starting the kick with the piston just over compression and kicking all the way thru? And no throttle.)

[Stratos23]

Just to try it if you haven't already: Change over the two wires connected to the inductive pickup.
Depending on design, there can be different behaviour, relating to the shape of the pickup waveform.
Sometimes, if the waveform is not symmetric, advance behaviour can be quite different if the wires are changed about.
It's a long shot, but you never know. Easy to try.

It is a long-shot Jordan, but makes sense - and easy to try! Thanks for the tip! I'll let you know if it makes any difference.

[Stratos23]

You are having a right merry time with this bike, Greg!

Err ... static timing for a 450 is 0°. Enuf to make a difference? Dunno ... I have read of a bike kicking back with just 1½ degree of advance, so maybe.

(You are starting the kick with the piston just over compression and kicking all the way thru? And no throttle.)

I know!! I can't remember a bike being so stubborn with one particular issue (and I have a few bikes!). And yesterday's gruesome incident was a real one off!!

But yes, I was aware that the std set-up on a 450 for starting (zero advance) was 0º BTDC - so I did try progressively retarding it to this point - and even further. It didn't seem to make a difference, but then that was with the coil it came with. I might try it all again once I get a beefier coil.

And yes, starting technique is as you describe (which works a treat on other Duc singles in my experience). Although with the kick backs being so violent, you can't help but be a little bit tentative on the full 'swing through' (which, ironically increases the chances of a backfire!!).

Was pondering as I went to sleep last night - do you think the 'wasted spark' system (ie the spark discharging on every stroke) would more likely cause a backfire (since the piston has to move through an exhaust stroke before it's likely to fire on the next compression stroke)? And does the std 70's Ducati Electronica system fire the same way? (they do on later 80's twins with Bosch ignition).

I'm still yet to hear of anyone using this Electrex system on a Ducati single? I guess the ultimate test is to swap out for an entirely different system. It still have the original points, but I've now rewired the bike for electronic ignition.

Hmmmm...!!

[graeme]

A mate used the Electrix World system same as yours on a 450 Scrambler.
He was telling me about the strage (to me) way they suggested to set the timing.
I think from memory it was advanced at start up which I thought was dangerous for reasons you understand.
I'll ask him how he set his.
He hasn't said it is hard to start.

The original '74 red box has a wasted spark.
Apart from the faint dots it's hard to set for starting and is best set for starting by trial and error. Once going set full advance to 28 degrees by strobe.

I have a 450 RT that has Saschse ignition
This is set at 0 to half degree BTDC using the LED on the on the ignition. Easy.
Advances are set by selecting one of 9 settings.
None of the advance curves are spot on for a 450 but it isn't a race bike and always starts easily.

Graeme

Ps I too have had the fold out lever fly off and hit the garage wall.
Getting the start timing set is most important for a 450.

[graeme]

Here you are, Barry said this worked for him and followed the Electrix World setting sheet.

"Hi Graeme, the Electrex world instructions say that using a timing disc, set the piston BTDC at max advance position (28 deg) and the line on the pulser must line up with the line on the rotor. When running I set the timing advance using a timing light. My bike has always started easily. Has the owner checked the timing using a timing light?"

Also

"It might be worth asking Electrex World why you set the static timing at max advance?
Regards Barry"

Regards
Graeme

[graeme]

He also said the spark was weak.
His Scrambler is close to stock and has standard compression.
If yours has high comp that may change where your timing should be set?

[ducwiz]

...

The original '74 red box has a wasted spark.
Apart from the faint dots it's hard to set for starting and is best set for starting by trial and error. Once going set full advance to 28 degrees by strobe.

...

Graeme,

sorry but here I do not agree.
Ducati used two different "red boxes" for the later singles, made by Ducati Eletronica or Motoplat. Both do not produce a wasted spark, as the pickup rotates with camshaft speed, so a spark is generated every 2 crank rotations. Turn the pickup rotor by 180°, and your bike wont start anymore.

Imo, setting ignition timing with a strobe light and a precisely made ignition pointer (ref. Ducati special tools) is absolutely mandatory.
Finally, the right starting procedure is also very helpful in avoiding back-kicking.

cheers Hans

[graeme]

Hello Hans
Yes you are correct, no wasted spark on Ducati E red box ignition.
Sorry for posting wrong information
Regards
Graeme