CDI unit blowing

[stephen.salvage]

I have had an elctronic ignition fitted to my Li 150 but the cdi unit keeps blowing after about a mile and I'm getting no spark from it, on my 3rd cdi unit now and at £45 a go the the mileage cost isn't very economical! Is there anything else I should be checking before fitting another?

[coaster]

Is it an AC or DC setup? I had a similar problem caused by a contact between the DC from the battery and the green wire to the cutout button. 3 CDI's blew, 2 Ducattis which blew a section out of the casing and an indian one that just died.

[petemaisey]

I agree, most likely a wiring loom fault or the junction box near the rear mudguard.

Pete

[JETEX]

Make sure your earths are good. The one on the flywheel cowling is up against a painted surface and is only earthing through the threads of the small bolt. I use another earth from one of the engine side casing studs to a clean part of the rear footboard support.

[coaster]

Just turning to basics for a moment, have eliminated the ignition switch by removing the green loom wire from the CDI?
Have you changed the plug, cap and ht lead
Have you checked that you have good resistance readings on both the LT coil and the pickup?

[stephen.salvage]

Looks like I need to check all of the above! Thanks for your help.

[coaster]

Looks like I need to check all of the above! Thanks for your help.

From that I assume that you are new to the art of Lambretta electrics? It's quite simple really, the stator plate is made up of 4 lighting coils (forget about them for now) and a Low Tension (LT) coil that provides the power for the spark. One side of this coil is connected to earth and the other to the green wire that emerges from the stator plate at the cdi end. A good coil reading taken with a test meter set to resistance should be between 450 and 500 ohms or thereabouts. The LT coil is the weakest link in the ignition system. There are no points in the electronic system so the energy from the coil is triggered by the little black pickup box that's mounted on the stator next to the LT coil. One end of the pickup is connected to earth and the other to the red wire from the stator, a good pickup should have a reading of between 95 and 115 ohms. The stator does not require any external power to operate (unlike a car) so as long as the engine is turning over, it will always generate a spark so in order the stop the engine, the ignition switch (or cut out button) shorts the ignition to earth. Indian type ignition switches are very prone to shorting out inside with the result that you ignition can be shorted out even when the switch is turned on. If you look at the connections on the cdi you will see that there are 2 green wires, one comes up from the flywheel and the other disappears into the loom and is connected at its far end to the ignition switch or cut tout button. Obviously, if the is a problem either with the switch of the wiring leading up to it, if the engine runs after removing the green to the loom it will tell you need a new switch or wiring......and you will need to stop the engine by turning the petrol off or stalling it

[stephen.salvage]

Couldn't ask for comprehensive help. I'm working my way through it! Thank you.