No spark back cylinder

[EVguru]

Can pickups partially fail or are they all or nothing?

They can in theory, yes.

The pickup is in effect a kind of alternator. The trigger waveform it generates increases in amplitude with rpm.
The ignition box fires when the waveform reaches a specific trigger voltage. If the voltage from the pickup at idle is too low, then the ignition won't trigger.

There are two seperate pickup coils and they can suffer from shorted turns, so the voltage might be too low at low rpm.

[Al B]

Thank you for the comprehensive replies


The collection of parts is:
A US imported 3 ½ sport. Electric starter, alloy wheels, painted mudguards, had a left hand shift, no bodywork, so I think it’s 1980

Red pickup – no visible signs of cracking. I haven’t disturbed it so the make is unknown
Blue Ducati labelled transducers (numbered 32398112) which I fitted just over a year ago.
When I got the bike it had grey 3rd gen ones , no stickers to denote usage

I changed the transducers as the red/grey combination is supposed to be an unholy match – though it seemed to run fine. It then ran fine on the blue ones.

The cam belt is also about a year old. The play is minimal and with the strobe on the front HT lead the mark is steady.

I’m fairly happy the transducer earths are good as I’ve had them apart whilst checking things over. I’d swapped the transducers around to see if that changed anything but to no avail.
Engine earth tests as good. The plugs and resistor caps are also a year old (and measure 5k) and I’m intending to get some new copper HT lead.

I’ll check the coil resistances and fortunately have a scope to hand so can do the running test too.


I’ve just looked again – properly this time – at the published ignition advance curves and my previous ideas were clearly amiss. At idle the advance is much more that I’d thought so the relatively little change up to 6k is probably about right. I’m too used to dealing with bob weights and vacuum advance!


The exhaust popping seemed to be pretty much in time with the lack of spark. I’ve not disturbed the carbs and it was running sweetly before.


First step then is new HT leads and static check the windings. Then I’ll try comparing the outputs of the two pickups whist running

Alan

[mbmm350s]

Hi Alan,

Thanks for the information.

What type of plugs do you have fitted,
as you have resistor caps its considered better to use non resistor plugs eg NGK BP7ES

The blue ducati transducers are OK.
The third series grey transducers (323982) will run with red pick up, but they add an additional diode in series.
so the ignition needs to be retimed as it will otherwise be retarded across the whole range.
[for clarity the black pickup works only with grey transducers without any modifications]

Good plan on HT leads, The Club Strada stopped running on front cylinder, cutting open the HT lead showed about 1 cm of corrosion.
Fortunately an easy fix.


Mark

[Al B]

So this evening I popped the tank off and replaced the HT leads. Whilst doing so I measured the resistance on the green wire...183 ohms so it's a bit on the low side.

The voltage when kicking the engine over was 70 or so according to a multimeter. I used the scope to capture the output and the peaks were well over 100V


Apologies for the ropey pics I can't find the software to download the plots off the scope!

I also looked at the pickup outputs to see if they were the same by popping the red wired off the transducers.



Having done all that and put things back together i hooked up the strobe onto the rear plug lead and started up. A nice smooth idle, no popping and no interruption in the strobe flashes.

I stripped the ends off the HT lead to checl the condition and they didn't look bad. A little bit of oxidation at the transducer end but only the first few mm. Enough to make a difference it would appear!

Thanks for all the information and assistance, now for a trial run and hopefully I'll be along to the meeting and AGM

Alan

[mbmm350s]

Hi Alan,

Often its the way that simply cleaning up the connections fixes things.

So this evening I popped the tank off and replaced the HT leads. Whilst doing so I measured the resistance on the green wire...183 ohms so it's a bit on the low side.

183 ohms is low, whilst the bike starts OK i guess you can just keep an eye on it and plan to rewind over the winter perhaps.

Was the scope trace with the green wire disconnected? you say when kicking over?

///////////////////////////////////////////////////////
For the benefit of future readers of the post.

Its instructive to measure the generator with the engine running at constant rpm with everything connected.
Please see my really scrappy drawing, this has annotations which are useful.

Generator.jpg (504.52 KiB) Viewed 5368 times

If you measure the waveform with the engine running you will see that it has 12 distinct phases in 720 degrees of crank revolution this is obvious because the generator rotor has 6 magnets If we take as our reference the passing of the magnet just after the rear cylinder has fired, we see an alternating waveform with positive peaks of around 100V at idle. This is not a conventional generator sinusoidal waveform because the circuit is driving a capacitive load. At cycle 8 we notice that the waveform is clamped to 1V, this is the front cylinder firing, the thyristor in the transducer triggers and shorts the green wire, thus discharging the capacitor of the front transducer and creating the spark through the CDI coil. The thyristor continues to clamp the generator until the generator voltage goes negative at phase 9. The rear cylinder fires at phase 12.
All the peaks should be the same, of particular interest is the peak just before cylinder 2 fires. A possible critisicm of the design is that the energy available to cylinder 2 is compromised and really a twin should have an CDI generator winding for each cylinder.

It is important that clamping only occurs in the phases shown, there is no wasted spark, that the clamp forward voltage is not more than about 1V. The negative peaks play no part in the ignition save to allow the thyristor to switch off. The transducers must never be operated from a d.c 12V source. Missing trigger will be evident. However the thyristor may trigger and discharge the capacitor but still there might be no spark. To see the spark needs the inductive pickup of the strobe gun (if you have it) exposed to the oscilloscope or some other inductive pickup. Now to increase the rpm and confirm that the waveform doesn't change in its form, the peaks should not collapse.
I can measure with some accuracy that the firing angle between cylinder 1 and 2 does not deviate from its ideal 286.7 degrees more than a few degrees.


Mark

[EVguru]

The forum crops the picture, right click and 'view image', or even save it.

The alternator waveform isn't sinusoidal because it hasn't been deigned to produce a sinewave. The capacitor is only charged on the positive half cycle, so there can't be any capacitive loading on the negative cycle and yet the waveform is quite symetrical.

I briefly ran a bike with full wave rectification of the ignition winding. I was also running without the copper shorting plate that limits the voltage as rpm rises (and gradually cooks the coil), instead using external voltage limiting. The combination acted as a rev limiter and as I was selling that bike it was restored to stock, but perhaps merrits further investigation.

[mbmm350s]

The alternator waveform isn't sinusoidal because it hasn't been deigned to produce a sinewave. The capacitor is only charged on the positive half cycle, so there can't be any capacitive loading on the negative cycle and yet the waveform is quite symetrical.

Paul is quite right, there is a diode in the transducer so only positive half cycles charge the capacitor.

The purpose of the crude drawing is really to show what the waveform should look like and what to look out for.

For later generators the copper voltage limiter plate is extended and riveted to the base plate, to facilitate this the generator
winding position was moved from the 1 o'clock position to the six o clock position. This may be able to help with heat dissipation ?
but do we have evidence that later generators are more or less reliable than earlier ones?

In any case please be assured the standard set up is very reliable,
the fact that generator failure is occurring now due to age related winding insulation breakdown (accelerated by temperature)
is not surprising.
It is a straight forward job to rewind the ignition winding I recommend using a professional rewind company with the appropriate experience.
A home rewind/repair has been done by many with success.


Mark

[EVguru]

For later generators the copper voltage limiter plate is extended and riveted to the base plate, to facilitate this the generator
winding position was moved from the 1 o'clock position to the six o clock position. This may be able to help with heat dissipation ?
but do we have evidence that later generators are more or less reliable than earlier ones?

It may have been done for heat dissipation, but the plate isn't an exteneded version of the shorting plate, it's a whole extra plate. This means more copper and more heat, so possibly even counterproductive.

I'm now seeing alternators where the coil bobbin itself has degraded from heat and they'll break whilst being removed. I've had some bobbins 3D printed in nylon.