Effects of a lightened flywheel

[MickYork]

...................A battery DC supplied CDI makes a good powerful spark right through the rev range. This help enormously with low inertia tickover and resists 2 stroke plug fouling. It also overcomes reliance on the shitty LT stator coils.

I suspect the reason this isn't adopted on a Lambretta will be down to the current draw on the battery. I have no figures but do know that many years ago when the dynamo on my mini packed up I had a flat battery within 20 miles or so I daresay it would be fine for open road use but slow town riding might soon flatten the battery

If the right critical component fails, then a ride home with the AA is the nature of the beast. It's all about detecting and improving the weakest link.
I use a DC powered, computer programmable race CDI on my Jet. It consumes 200mA of current. That's 2.5W near as damm it. (they all consume around this figure for single cylinder applications) Even with my puny 2.1AH battery to power it, without charging, I reckon I'd easy get a thousand miles before it quit.
Charging a flat battery overnight, at some roadside garage, in the pissin rain, in some godforsaken place that doesn'y speaky the Queens, is much easier than trying to fix a knackered stator.
During the day, I could run my ignition from a solar cell!

Can you give some more info' on the dc cdi (cost, where to purchase, etc).
I've had a quick look and some of the ones I've seen have a pre-set map inlaid into them, for a specific motorcycle, not ideal for a Lambretta.

[Fast n Furious]

Ultimately it is necessary to make some mods to your wire end terminals but before doing that you can make up an an electrical adapter to convert directly from your original terminals allowing you to trial the conversion before committing to a more permanent solution.
The photo below is of a generic CDI fitted to most GY6 derived twist and go's etc.

GY6 CDI.jpg

These units are available for AC sourced, magneto driven CDI's like your standard Lambretta's and Vespa's.
They also come as DC sourced for machines that are a bit more modern.

The AC ones can be used on a standard Lambretta stator sourcing the required 200VAC power from the crappy LT coil. No advantage really, except, like in the photo some have and adjustable ignition map via potentiometer adjustment.
The DC ones need a DC supply. This could be sourced from your battery if you've got one but you would need to put a switch in line to turn it off when not in use just like a motorbike does. Not very Lammy.
You could use a simple low power rectifier/ regulator connected to the lighting circuit to provide the DC for the CDI but this can be problematic on tickover with the lights on.

The best way to do it is to use an old Ducati points Stator converted to 12v. Dump the points and fit a pickup trigger coil. The LT ignition coil on a points stator is made of much sterner stuff and is designed for low voltage inductive ignition. A simple DC regulator rectifier on the green wire provides all the 12VDC the DC CDI needs even when just kicking over.
To kill the engine you just need to connect the green wire from the ignition switch to the pickup trigger line.

So, what about the ignition coil?........ There is a minefield of them available. Make sure your choice is for Capacitive Discharge Ignition and NOT Inductive Ignition. Just avoid anything from the far east. Far east manufacture and high voltages don't go well together. This is one area where you can't skimp on price and quality.

I use a coil from an American outboard motor. These are very good quality and designed to work in wet and damp environments. There are shedloads available on auction sites new and used. If you found yourself needing a spare then these are available from most Marine Chandlery's. There are a lot more of these around the world than Lammy dealers!

Use copper HT lead. Non suppressed Plug cap (The red Jap one with 3 letters on the top is perfect) and a resistive suppressed plug.

These CDI's typically cost less than a pack of cigarettes when buying directly from China, so its worth having a go.
There are many variants of DC CDI's out there. The best and most expensive ones are setup using a laptop and are adjustable "on the run", which makes the task of optimising the ignition timing a doddle.

Mick.... PM me if you want to see one working.