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UPDATE: The problem was with the stator coil as detailed in a later post.


Looking for advice and experience with faulty ignitions.

I've been stranded a few times this month by an intermittent problem. The bike runs fine, but then I park for a short time and it will not start again. There's no spark and it's not a problem with the spark plug (which is a nice tan colour). I let it sit overnight and the next day it works fine like nothing was wrong.

I followed the ignition checks in this guide and all was within the acceptable range:

https://www.pedparts.co.uk/blog/piaggio-ignition-fault-finding

The guide recommends replacing the CDI when the bike passes all the other checks. Yet, I suppose it could also be due to a faulty ignition switch.

I'm fine with replacing the unit, but I'd like to be reasonably sure that the CDI is the source of the problem so that I'm not stranded somewhere again.

Advice? Thanks!
Vespa PX CDI Ignition Unit
Vespa PX CDI Ignition Unit
⚠️ Last edited by robertitaly on UTC; edited 1 time
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Should be easy enough to rule out the ignition with a multimeter. But why would that be intermittent in a way that gets better if you leave it a while? Surely you would find turning the key on and off would sometimes work. But it just doesn't seem like a part that would act that way.

Could also be something weird like engine getting hot causing some play in the crank and the ignition coil no longer lines up properly.

I'd be replacing the CDI both as the most likely problem and because at this stage probably far easier than trying figure out if it's something else or not.
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That brand of CDI is crap. Common issue with those. Get a Ducati CDI.
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Before you buy a new Ducati CDI, try this simple test: disconnect the last green wire (with plastic terminal cover) on the CDI, and see if the problem goes away.
If it does, you may have an intermittent short from that wire to ground somewhere in the frame.
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sp949 wrote:
Before you buy a new Ducati CDI, try this simple test: disconnect the last green wire (with plastic terminal cover) on the CDI, and see if the problem goes away.
If it does, you may have an intermittent short from that wire to ground somewhere in the frame.
He did that when he followed the ignition checks in the guide from the link he provided.
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sp949 wrote:
Before you buy a new Ducati CDI, try this simple test: disconnect the last green wire (with plastic terminal cover) on the CDI, and see if the problem goes away.
If it does, you may have an intermittent short from that wire to ground somewhere in the frame.
Your suggestion is valid.
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My own testing would have been:

1) Run a wire from the last terminal to the ground (forget about the green wire that comes from the switch/ignition at this point)
2) If the scooter dies when you disconnect the wire from the last terminal to the ground then you know the ground on the CDI is bad.
3) Now do the same thing but with the green wire that comes from the kill/ignition. Try killing the scooter first with the switch. Then try killing it by disconnecting the wire directly from the last terminal. This test will tell you what section of the ground is faulty
4) Do an ohm check on the spark plug wire.
5) How is the spark plug cap?
6) How is your ground overall (round flat head screw and metal engine shred?
7) Have you tried a test light on CDI terminal and ground?
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Thanks! The comments have been very helpful.

The problem happened again this weekend. I stopped at a grocery store and it would not start afterwards. A faulty ignition switch seemed unlikely, but I disconnected the green "kill" wire at the CDI just to make sure. That made no difference. Yet, for some reason there was no spark coming from the HT coil.

After waiting an hour the motor started and ran normally. That's weird, but perhaps the faulty electronics just needed time to cool and reset.

I'll replace the CDI since that seems the most likely problem.
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robertitaly wrote:
Thanks! The comments have been very helpful.

The problem happened again this weekend. I stopped at a grocery store and it would not start afterwards. A faulty ignition switch seemed unlikely, but I disconnected the green "kill" wire at the CDI just to make sure. That made no difference. Yet, for some reason there was no spark coming from the HT coil.

After waiting an hour the motor started and ran normally. That's weird, but perhaps the faulty electronics just needed time to cool and reset.

I'll replace the CDI since that seems the most likely problem.
Roberto,
how old is your PX 150? Mine is 1985 USA model, and have had it since new. 10 years ago, I was having the same problem you have now, no spark after running for awhile.

After making all the troubleshooting checks you did, I removed the stator plate under the flywheel, and found that most of the wires to the coils had lost their insulation, especially the green one from the white wrapped ignition charging coil. So, I repaired that green wire and all the other wires the best I could. The problem was resolved, but not for very long. I end up replacing the entire stator plate, and since then, no more ignition issues.
The engine heat is the biggest killer on the engine wiring on these motors. You may want to inspect your stator plate, if it is still the original one, before buying that CDI.

Ciao, e buona fortuna dalla Florida!
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Have you performed the testing when you had a no start condition. In other words while still hot with no spark? I've had the same issues and it was my CDI then, but like you I was a bit nervous about reliability.

I want to order one of these, I like the diagnostic led.

https://www.scootronics.co.uk/shop/CDIs-c72309451

He mostly does Lambretta electronics but has a few universal parts.

https://m.facebook.com/scootronics
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sp949 wrote:
Roberto,
how old is your PX 150? Mine is 1985 USA model, and have had it since new. 10 years ago, I was having the same problem you have now, no spark after running for awhile.
<snip>
Grazie, that was helpful. Mine is an Italian model year 2000, but I think the wiring is essentially the same. The external wires are in perfect condition but I should check the internal wires (under the flywheel) as you suggested.
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Guessing CDI or stator.
Coils expand (and can disconnect) when hot. Reconnect when cooled.
Takes a long time for an engine to cool off. Mine maybe a half hour.
Same happened to my 80's Grande, FWIW
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Christopher_55934 wrote:
Have you performed the testing when you had a no start condition. In other words while still hot with no spark? I've had the same issues and it was my CDI then, but like you I was a bit nervous about reliability.
<snip>
Good point. It's intermittent and I was only able to preform the diagnostics when I got it back home and it was working again. Regardless, I would have expected to see some borderline values that indicated a problem, but everything was spot on.

I found a schematic of the CDI unit, which is actually quite simple. It charges a capacitor through a diode and then discharges it through the coil when the SCR switch is triggered. And the problem just "feels" like a failing SCR that gets worse with heat.


Anyway, those CDI units with the status light are cool and not much more expensive.
Ignition Parts Schematic
Ignition Parts Schematic
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I had exactly the same thing on a PK75. Would run one day but not the next. Changed the stator as wiring corroded and it all came back to life.
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ferriswolf wrote:
I had exactly the same thing on a PK75. Would run one day but not the next. Changed the stator as wiring corroded and it all came back to life.
Thanks! It seems you and sp949 are right about the stator. I installed a new CDI unit today and was disappointed when the motor died just like before. Yet, this also allowed me to test the exciter coil with a hot engine and the green-white wires were an open circuit (fail).

After more research, it seems these Vespa stators have a life span of around 20 years. And if the ignition problem is heat related then it is usually the stator and not the CDI. In retrospect, the stator has been slowly failing for 6-months, but it finally worsened to the point that I noticed it.

Anyway, I'll post the result after I replace the stator. Hopefully that will help others because this seems like a common problem with older PX models.

Thanks again everyone. Your comments were all very helpful.
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Well my stator was 34 years old. It has ducati cdi and having done all the multimeter tests it all pointed to stator. You could still get a spark which was confusing but must have been very weak. Managed to get a new old stock four pole motovespa stator as they are different to most. Lights are permanently on and no electric start.
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robertitaly wrote:
Thanks! It seems you and sp949 are right about the stator. I installed a new CDI unit today and was disappointed when the motor died just like before. Yet, this also allowed me to test the exciter coil with a hot engine and the green-white wires were an open circuit (fail).

After more research, it seems these Vespa stators have a life span of around 20 years. And if the ignition problem is heat related then it is usually the stator and not the CDI. In retrospect, the stator has been slowly failing for 6-months, but it finally worsened to the point that I noticed it.

Anyway, I'll post the result after I replace the stator. Hopefully that will help others because this seems like a common problem with older PX models.

Thanks again everyone. Your comments were all very helpful.
Hopefully testing hot and finding on open will put an end to the issues. Nice to actually find an issue, makes you feel better about reliability when it works.
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UPDATE
Replacing the stator fixed the ignition problem. The stator was 20 years old, but there was nothing visibly wrong with it. The wires and connections looked fine. However, when hot, the exciter coil was an open circuit (green-white wires). So the problem might be hidden within the coil wrappings. Hopefully this info will help others in the future.

The stator is relatively easy to replace. It can be done with basic tools with the exception of the flywheel puller. During reassembly, make sure the stator is rotated to the same timing mark as the original and careful that the Woodruff key doesn't fall out as you slide the flywheel back on to the shaft.

The photos show the old and new stator.
Vespa PX stator -  Exciter coil and sensor
Vespa PX stator - Exciter coil and sensor
Vespa PX stator - new
Vespa PX stator - new
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Someone else here had a similar problem recently. Running and then cutting out.

Cant remember who, but i remember mentioning they should check timing (which would make sure your stator is working properly)

Its great they can refer to this...
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