Won't run - electrics?
 

Wah, my 34L is driving me nuts. After 25 years of (almost) blameless behaviour, its finally decided to play up bigtime.

Symptoms: starts fine, runs for a couple of minutes, then cuts dead with no warning. Will immediately re-start (first kick), and run again for an indeterminate (but short) time, then cut once more. Repeat 3 or 4 times, then she dies completely and won't be revived for a few hours. Cuts are (usually) accompanied by an enormous backfire (which its never done before).

I was having some grief anyway with a sticky carb float, so have replaced float + fuel valve/seat and blown out all the passages with compressed air. Float height/fuel level is now perfect. Loosened fuel cap, removed air filter to see if there's any strangling of air or fuel. OK.

The 'instant death' symptoms suggest to me that its an electrical fault (there's no gradual leaning off or decline/increase in revs which you'd expect from fuel problems), but I can't imagine what electrical fault would do this consistent stop/start/dead behaviour?

Next job is strip off the wiring loom and check all the continuity/insulation, I guess? Wiring loom was replaced about 10 years ago, btw, and I'm running a big capacitor in place of the battery, if that's relevant.

Any ideas gratefully received!

What's the spark like?
It sounds as if something electrical is breaking down when it gets hot.
If it was my bike I'd replace the plug cap, then coil,then CDI and finally get the generator tested.
You should be able to get a used coil cheaply enough.
Mark

OK it does sound a tricky one but if you're not that good with electrics a good auto electrician who has the right tools not least that powered screwdriver/ prodder that puts 12 volts where they may be non can save you a lot of time and money, thats my experience anyway, good luck with it.

could be many things but do a quick check on the charging out put

Sparkplug gap is too wide!

Spark plug is correctly gapped and brand new. Generator is fine (at least, the neutral light comes on nice and bright when I kick it), and was rewired 10 years ago with few miles since.

Coil failing with warmth sounds most plausible ... I'll get onto that and report back ... hope its not the CDI!

Most likely
 

This is typical of an electrical component getting too hot as current passes through it, then failing. A bit of time to cool down and away it goes before failing once more. Suspect components are: ignition coil; CDI unit; CDI supply coil; CDI signal generator. It is possible it may be a duff connection so that's always the best - and simplest, and cheapest - place to start. In reality, the only way to check the other components is by replacement. It won't be anything to do with the charging system as all it does is maintain the battery.
Good luck,

Geoff

Fixed
 

Yay! Back on the road again :thumbup1:

I replaced the coil and plug cap with one from Kedo (for the princely sum of 15 Euros) and everything is once more perfect in my XT world.

Thanks for the input guys - much appreciated, as ever,

Martin