125 WITH NO SPARK

Lambertini era singles (125 and 250 - half v-twins)
mbmm350s
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Re: 125 WITH NO SPARK

Post by mbmm350s »

Hi Bill,

I sense your frustration from a far.

I do agree with others that the first suspicion has to be the generator winding.
I am not sure I have definitive data on 125H for the winding resistance (green to earth with ignition switch ON) as new,
for all others it is supposed to be 300 ohms, but in practice on the twins they work until the resistance
has fallen (due to shorted turns) until around 200 ohms.
The insulation breaks down due to heat, turns become shorted and layer to layer shorts occur which reduce the number of effective turns and so reduce the voltage being sent to the transducer.

Previously Paul mentioned how you can measure the a.c voltage (green to earth) which should increase linearly with rpm until it plateaus.

In order to remove any fault associated with the lighting coils you would have to disconnect the yellow and red wires from the stator- assuming your later 125 has the non direct lighting system.

Please don't let the rotor overheat, the magnetism could be damaged as well as further damage to insulation.

Mark
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Re: 125 WITH NO SPARK

Post by EVguru »

mbmm350s wrote: 21 Sep 2021 12:12 for all others it is supposed to be 300 ohms
I do wish this 300 ohm misinformation would go away.

That may be what is written in the 'blue book' and it may well have been true at the time of writing, but it's a physical imossibility for the overwhelming majority of coils (I have come across a small number of factory 300 ohm coils).

The resistance is a function of the number of turns and the resistance of the wire per turn. The resistance of the wire is in turn a function of its diameter.

I've measured the wire diameter on nearly all coils as 0.15mm (overall diamater is a little larger because of the insulation) as shown in this picture;
coil.jpg
coil.jpg (151.03 KiB) Viewed 955 times
With wire of this diameter, there simply isn't room on the bobbin to wind enough turns to get to 300 ohm. 300 ohm coils do exist, but they use thinner wire and I suspect were more prone to breaking down, which is why the thicker wire was used.

The resistance doesn't matter very much, just the voltage ouput (within reason).

If you KNOW that a coil was wound as a 300 ohm coil, but it now measures 220 ohm, then you know it has shorted turns and is failing.

If that coil was wound as a 220, then it's proably just fine, but many a good coil has been condemned and needlessly rowound (sometimes badly) just because it read the 'wrong' resistance. People have often used thinner wire just to be able to achive the 'correct' 300 ohm value.

When I rewind a coil, I don't bother counting turns, I just use calipers to meaure the size of the coil. That will usually get me to withing +/- 4 ohm of the nominal 220. I will write that value on the coil insulation, so that in future, someone can see if the value has changed.

The value will be that of the coil at room temperature. It should increase somewhat as trmperature increases. If it drops when hot, then that is an indication that the insulation is breaking down and turns are shorting out.
Paul Compton
http://www.morini-mania.co.uk
http://www.youtube.com/user/EVguru
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Ming
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Re: 125 WITH NO SPARK

Post by Ming »

I think 300 ohms has entered into Morini folklore, Paul, despite your frequent explanations. Having rewound a coil myself (using 0.15mm wire IIRC), I seem to remember a nominal 240 ohm result and a rather bulging coil. It fitted, though, and the ignition was fine afterwards. Not a difficult job, the hardest thing was finding a suitable coil former.
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themoudie
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Re: 125 WITH NO SPARK

Post by themoudie »

Good morning Gentlemen,

Resistance between the green output cable from the low tension ignition coil and the stator ring is 478 Ohms, on my stator.

Resistance between the red output cable from the black magnetic trigger coil and the stator ring is 110 Ohms, on my stator.

Both readings are taken at an ambient temperature of ~15°C, on the 2000 Ohms scale of my Gunson multi-meter, with the stator removed from the engine.
I was getting very similar resistances when the stator was on the engine.

My conundrum at present is whether to retain the black magnetic trigger coil, original with the stator and the #323921 transducer. Or, fit a red magnetic trigger coil that apparently has a diode in its circuit, as this has proved successful for members of the German Morini Owners forum, when used with transducer #32398 112, the replacement transducer that I intend to use. The Dutch website table of transducers/pickups shows for the 125H, "125H, #323921 (green sticker), no pick-up, but a sensor in the alternator". However, my 125H engine has transducer #323921 and a black square magnetic trigger coil attached to the stator pole to the right of the low tension ignition coil. The 250T, according to the Dutch website table shows "250T (single), #323931 (?? sticker), pick-up, red". Hence my quandary, as the transducers are the same type, but the 125H trigger colour is not given and mine conflicts with the information given for the 250T!

Should I order a red trigger coil and include that in a rebuild?

I note from the circuit diagram drawn for the "self-build" transducers that there are two variables:

One, being a diode #1n4007 in the red cable from the magnetic trigger coil, if this is the black colour type. I assume that this diode is contained within the circuitry of the red colour trigger coil?

Two, the second resistor on the red wire input to the transducer has a value of "68 Ohm", I understand this is if the transducer is to be used on a 350 V-twin engine, with an advance of 34°BTDC and 2° less than recommended for the 125H (36°BTDC @6,000rpm). Whilst if the transducer is being built for a 500 V-twin engine, with an advance of 30°BTDC the resistor has a value of "270 Ohm". However, is the value of this resistor in the circuitry of my replacement transducer #32398 112 known, or will it be a case of "suck it and see" experimentation on my part on the rebuilt stator?

Thank you all for your contributions.

Bill
Attachments
Dutch Moto Morini website published self-build replacement transducer circuit diagram.
Dutch Moto Morini website published self-build replacement transducer circuit diagram.
circuit.jpg (114.08 KiB) Viewed 932 times
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Re: 125 WITH NO SPARK

Post by EVguru »

The 125 has some unique features. The ignition is a crankshaft triggered 'wasted spark' system and the tachometer cable drive is from the right hand (timing) side.

All the other air cooled V-twin based engines equipped with Capacitive Discharge ignition, EXCEPT the 125 single have camshaft mounted pickups.
The later Kokusan ignitions are Inductive Discharge and use pickups external to the rotor. They are also wasted spark.

References to black and red pickups and the presence or not of a diode only applies to the camshaft mounted types. The 125 pickup is part of the alternator and may well be the same as a Vespa part, but I've never had an igntion probelm with a 125 and had to investigate.
Paul Compton
http://www.morini-mania.co.uk
http://www.youtube.com/user/EVguru
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themoudie
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Location: Perthshire

Re: 125 WITH NO SPARK

Post by themoudie »

Thank you for your reply Paul.

Does that then mean that I don't need to concern myself with the type of Vespa/Lambretta trigger coil that I use, as they are available as both black and red units? :?:

Does your suggestion of using electrical resistance to control the advance, rather than the angle of the magnetic trigger coil in relation to the crankshaft/camshaft axis still hold true? :?:

Or, am I going to have to mechanically retard the timing by 12° beyond the factory set ignition timing point, so that I can acheive the recommended ignition advance of 36°BTDC @6,000rpm as recommended by the factory? :?:

Thank you for your time, Bill
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