CDI Conundrum

[coaster]

Desperately seeking views on my CDI problems. My problems seemed start 50 miles into the Coast to Coast ride last October when I stopped for breakfast on my way up to Whitby on the Friday. Ran fine up to that point but no spark when I went to re-start it. Put on my spare which had been kicking around in the toolbox for 5 years and got to my B&B in Robinhood’s Bay with no problem. However, it wouldn’t start later in the evening , no spark again.
The following morning I managed to get hold a brand new Ducatti CDI off another rider and I was back in business. No more problems for the rest of the ride across and all the way back to Norfolk until 2 weeks later on the way to the shops it just died whilst I was ridding, took the CDI off and found this:

I then took the old Indian CDI off my other scoot that had been good for around 6k miles and all was good again. Then 3 weeks ago I changed the complete engine and doing plug chops it refused to re-start…..no spark again, fine before switching off and then nothing.
At this point I was convinced that the problem was vibration related so on Saturday I made a rubber mounted bracket and fitted another brand new Ducatti CDI but after less than 20 miles this happened

I put another Indian one on for the moment but I’m at a bit of a loss, the problem seems to persist even with a different engine. Also there might be 2 different issues going on, both Ducatti CDI’s have literally exploded with the engine running whereas the Indian ones have just stopped working with no visible external damage.
HELP

Colin

[J1MS]

I would check earth continuity between cdi frame and engine.

High resistance can cause cdi units to fail.

Also high resistance at the plug and suppressor cap.

I think innocenti reccomend no higher than 5 or 10 k Ohms for the plug and cap.

You could also remove any carbon deposits on the spark plug thread and cylinder head plug thread.

Best of luck.

[coaster]

Hi J1MS, I was hoping that you and or firekpd would respond :bouncing: I'll check the earths, the HT lead and cap are new. I've been racking my brains and was wondering if it's possible bearing in mind the my scooter has a full DC conversion, if a stray feed from the DC circuit onto the ignition earth wire could cause the problem :wtf: I did have a short in my loom last year on the way back from the Euro which caused some melting of insulation and the earth wire from the CDI to the ignition switch was on of those affected. I'n not sure of the interrnal circuitry of the CDI, coil, capacitor and thyristor? What doi you recon?

[jason frost]

Have you tried running with the green wire from the switch out ? its below where they plug in

[coaster]

Have you tried running with the green wire from the switch out ? its below where they plug in

I left it unplugged for the 4 mile ride home and I am now highly suspitious of it, I'll do some testing at the weekend

Cheers

Colin

[J1MS]

The CDI looks to have shorted to earth at the point that the cdi generates a spark that's nearest to the frame. ?
This Woukd only happen if a high resistance to the spark at the plug end is high either by a bad earth cdi to frame and engine.

Do you use the thick braded wire for engine to frame earth. ?

When a cdi has power shorted back to it from a dc sorce it can do damage but it wouldn't or shouldn't explode a capacitor.

[coaster]

Do you use the thick braded wire for engine to frame earth. ?

No, just regular wire as used in the loom

[J1MS]

A very high voltage makes a return path to earth through that wire.

The high voltages are of short duration, any extra resistance to earth through a low amperage earth wire will cause the box to try and find a path with a lower resistance, so my guess is this could be the probable cause.

[coaster]

A very high voltage makes a return path to earth through that wire.

The high voltages are of short duration, any extra resistance to earth through a low amperage earth wire will cause the box to try and find a path with a lower resistance, so my guess is this could be the probable cause.

Many thanks, I will be replacing my earth wires with heavy duty braided cable at the weekend.....and after yesterdays debacle getting stuck in the forest 4 miles from home I will make sure I don't go out without my AA card and or spare CDI