First ride, first breakdown

[Romomoto]

So - after starting my newly acquired 1966 Mk3, I went through the typical preparations for its first contact with the road in 20 years. Then I fueled it up, kicked it to life and ran it down the road. It ran fine, even idling (kind of) though needing some fettling and fine tuning, for about three miles before it started to exhibit a misfire. I pointed it toward home and chugged along in second, down on power and misfiring. The misfire grew progressively worse until the engine shut down.

I got a tow home, pulled the plug and found no spark. I found that the non-stock NGK resistor cap had gone bad - open circuit - so I replaced the plug wire & cap with a known good one.
Still no spark.

The Mk3 has no battery, and the coil is powered directly from the generator. I hooked up a 6-volt battery to the coil and spun the kicker; no spark. I removed the + connector from the battery and got a nice fat spark as the field collapsed. So the coil is good, the replacement wire and cap are good.

Along with the + power source, the #1 terminal on the coil is wired to the points and to the kill switch. The points short out the coil when closed. I do not understand this scheme; removing power from the coil collapses the field and causes a spark. Shorting out the coil does not.

Can someone explain to me what might be going on here?

Also, I found that the insulation around the wires from the generator out through the engine case is crumbling and exposing the wires, so I need to replace them. Which of course means pulling the clutch and the stator. Anyone know a good source for sheathed 3-conductor wire to replace the original?

Thanks in advance,
Romomoto

[George]

https://www.classicducati.com/parts-sho ... d-lighting for replacement 3 wire

[themoudie]

Aye Romomoto,

Not sure where you live, but I use Vehicle Wiring Products in the UK for all my wiring supplies. Here is the link: Vehicle_Wiring_Products

Electricery is not my forte, but from you description of the misfiring and eventual demise of the spark, I would be checking every part of the charging and ignition separately and from your comment about the wires from the generator, fear that it may have shorted, with each of its coils needing investigation.

Sorry not to be more positive, but 'original' Italian wiring is often about as much good as multi-coloured spaghetti at conducting electricity!

I hope that others with specific knowledge of your machine can help with the fault diagnosis and repair.

Good health, Bill

[Romomoto]

Thanks, Bill and George. I should have mentioned that I'm in the US, so a supplier on this side of the pond would be advantageous.

[Bevel bob]

I would fit the Monza alternator ,coil and advance retard unit with a battery.

[Ventodue]

<snip> Along with the + power source, the #1 terminal on the coil is wired to the points and to the kill switch. The points short out the coil when closed. I do not understand this scheme; removing power from the coil collapses the field and causes a spark. Shorting out the coil does not.
Can someone explain to me what might be going on here?

Ok, I’ll do my best …

“The points short out the coil when closed.”

No, they don’t.

Instead what they do when closed is complete the circuit of the primary, aka low tension, winding in the coil.*

When they open, this circuit is broken. This act of breaking the low tension circuit induces a charge into the secondary, aka high tension, winding in the coil. Now, because this secondary winding contains many more turns than the primary winding, this charge is magnified several thousand times over what was in the primary.

Result is a high tension (= high voltage) charge in the secondary winding.

And precisely because it IS high tension, this charge can leap a gap to get to ground. And when it leaps a gap, it creates a spark.

So, connect the high tension winding to a spark plug threaded into a cylinder head, and vroom! We’re burning gas!

* A bit of technical stuff: The electricity that flows through this circuit creates a magnetic field within the coil and it is the collapse of this magnetic field that in fact creates the charge in the secondary.

But frankly, you can ignore all that. Just say thanks to Michael Faraday every time it happens …

HTH

Craig

[Ventodue]

<snip> Still no spark.

The Mk3 has no battery, and the coil is powered directly from the generator. I hooked up a 6-volt battery to the coil and spun the kicker; no spark. I removed the + connector from the battery and got a nice fat spark as the field collapsed. So the coil is good, the replacement wire and cap are good.

Check the points and the condenser, aka capacitor. Here's why ...

If the points aren't breaking the primary circuit, the primary winding won’t discharge. That in turn will mean that no charge will be induced into the secondary winding. That means no spark (see my post above).

But the fact that you got a spark when you manually did the points’ job - by removing the feed from the battery to the primary - shows that both the primary and secondary windings are ok.

A failed condenser can/will also cause the system to not work, for reasons I won’t go into here. They're difficult to check properly without specialist equipment, but a rough and ready test is to connect one terminal of a 6 volt battery to the condenser lead and the other terminal to its case. Only do it for a few seconds. Then bring the lead close to the case. You should see a little spark jump between the lead and the case.

In practice, no-one really used to bother with any of this because for the few pennies they cost it was just easier/safer to fit a replacement and be done with it. Dunno these days, tho' ...

[Romomoto]

Ventodue: Thank you for your detailed response. I understand how the coil works, as well as points and condenser.
What I did NOT understand is the way these components work together in this particular ignition system. I'm accustomed to the more standard systems and did not realize that this system is different. (I know, I really should get a manual, and am looking into that.)

I will check the condenser and replace if necessary.

Meanwhile, I need to replace the wiring from the magneto, so need to pull the clutch and stator. Any tricks I should know about this process, special tools etc. or is it straightforward as it looks to be?

Cheers,
Romomoto

[Bevel bob]

Clutch is pretty easy ,keep all plates in same order if possible. Watch out for loose spacer and washer on kickstart shaft. Alternator rotor must have the right puller otherwise you will ruin it. Rotor position on shaft critical with mag ign bikes to produce the power at right point of ignition. With the Monza battery ignition this is not important (thats another reason I would switch to it!!).

[Romomoto]

I guess once I have the clutch off I will see what I need to pull the rotor.
Is the rotor not keyed to the shaft? If so that's good to know.