Commando Cutting out

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Stillreel

Stillreel

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Took the 73 Commando out for a good long stretch today and its running fine. Just as I'm approaching the bottom of the hill (rather than the top), it stops dead; nothing. Switch to reserve: dead, no lights no spark.

Pull over and check the battery and there's a solid 12.30 V (and 13.39V when I got it running again). After 15 minutes poking and prodding, it starts right up.

Its a newer harness but the issue has got to be a short somewhere.

Where to start? Any suggestions?
 
Not necessarily a short in the system. That would likely show itself as a blown fuse. Start with cleaning up the ground(s).
 
Check those pesky handlebar switches!!!
Then have a look at the coils.

...then play with the carbs, as we all know that most electrical issues turn out to be carb related.
 
The symptoms point to an intermittent connection somewhere, check/bypass the kill switch, then move onto the key switch, ignition etc...
 
You said a new harness. Did you re-use the old connectors or did the new loom come complete with connections?
 
As noted, clearly an electrical issue per the description of symptoms - after it quit you had no lights and no spark. As noted, the grounds and a poor connection at the battery at either terminal is high on the list of usual suspects. Since you lost BOTH lights and spark, it has to be something common to both circuits. For example, a failed kill switch will not affect the lights, etc...

I'd look at the battery terminals, the grounds, and the ignition switch terminals/connections. Faulty ignition switch itself could also cause the problem.

Good Luck!
 
concours said:
Fuse holder
Click to expand...

I had the fuse holder causing intermittent cutting out on a new Andover Norton harness, the soldered end caps inside were not making full contact with the fuse so replaced it with a modern fuse and holder and the problem went away.
 
Thanks all. I think (I hope) I found the issue; spark plug lead was chaffing against the head steady and there was significant evidence of wear. Tidied it up and so far, so good.

Re: fuse holder. I replaced the fuse with a circuit breaker last year and that was my 1st thought. But it appears to be okay.

Thanks again.

BC
 
Stillreel said:
Thanks all. I think (I hope) I found the issue; spark plug lead was chaffing against the head steady and there was significant evidence of wear. Tidied it up and so far, so good.

Re: fuse holder. I replaced the fuse with a circuit breaker last year and that was my 1st thought. But it appears to be okay.

Thanks again.

BC
Click to expand...
Hi BC,
Spark plug lead as in from coil to spark plug? Would that cause the electrical outage you describe; i.e. no lights etc?

Ed
 
Stillreel said:
Took the 73 Commando out for a good long stretch today and its running fine. Just as I'm approaching the bottom of the hill (rather than the top), it stops dead; nothing. Switch to reserve: dead, no lights no spark.

Pull over and check the battery and there's a solid 12.30 V (and 13.39V when I got it running again). After 15 minutes poking and prodding, it starts right up.

Its a newer harness but the issue has got to be a short somewhere.

Where to start? Any suggestions?
Click to expand...
I had the same issue with '74 Interstate. Problem was blockage inside the gas cap (rust). Tank was getting vapour locked. Stuck compressed air nozzle into gas cap vent hole and cleared it out. No troubles since. Hope that's your solution!
 
APRRSV said:
Hi BC,
Spark plug lead as in from coil to spark plug? Would that cause the electrical outage you describe; i.e. no lights etc?

Ed
Click to expand...

I think so; I have the Boyer ignition on it which supplies intermittent spark to both sides simultaneously. If it was shorting out there, I think it would knock the whole system out. I'm not entirely convinced that this is the problem but in going through all connections and grounds, this did appear to be the most egregious fault, and so far so good; it ran well on the way into work today.
 
Stillreel said:
I have the Boyer ignition on it which supplies intermittent spark to both sides simultaneously. If it was shorting out there, I think it would knock the whole system out.
Click to expand...

Stillreel said:
Switch to reserve: dead, no lights no spark.
Click to expand...


An HT short would be unlikely to cause the lights to stop working.

For one thing, once the engine has stopped there would be no more HT short, for another, the HT current earths/grounds across the plugs normally so fully grounding an HT would only prevent that particular plug from sparking.

Power for lighting and ignition go their separate ways from the ignition switch so for both systems to lose power at once suggests the fault is more likely a bad connection somewhere between the battery and ignition switch (brown/blue), in the ignition switch itself or somewhere in the return (red) wiring that connects to battery(+).
 
Still worth opening up ign switch...we see threads here showing how nasty the contacts become and how to rejuvenate them.
And, if you have several keys hanging off the keyring, maybe the downhill creates enough of a pull on the keys to trigger open circuit in main switch.
 
L.A.B. said:
An HT short would be unlikely to cause the lights to stop working.

For one thing, once the engine has stopped there would be no more HT short, for another, the HT current earths/grounds across the plugs normally so fully grounding an HT would only prevent that particular plug from sparking.

Power for lighting and ignition go their separate ways from the ignition switch so for both systems to lose power at once suggests the fault is more likely a bad connection somewhere between the battery and ignition switch (brown/blue), in the ignition switch itself or somewhere in the return (red) wiring that connects to battery(+).
Click to expand...


I'm wondering now if the short drained the battery (thus no lights) and subsequently recharged itself while the bike was stopped. There was a 40 minute interval between stop and voltage check on a new(er) battery. Still a bit baffled to be honest but its running now.
 
Stillreel said:
I'm wondering now if the short drained the battery (thus no lights) and subsequently recharged itself while the bike was stopped. There was a 40 minute interval between stop and voltage check on a new(er) battery. Still a bit baffled to be honest but its running now.
Click to expand...

Unlikely, I would have thought because the spark energy is momentary and goes to 'ground' regardless of whether it jumps the plug gap or not as already mentioned, so extremely unlikely that would drain the battery and besides the HT shorting would have caused a misfire or that cylinder to cut out.

It's possible a heavy drain on the battery could have caused the ignition to cut out but it's unlikely the lights would have completely gone out as the ignition would have stopped working before that happened.

As far as I can tell it lost all battery power for some as yet, unknown reason but I very much doubt it would have been caused by an HT wire shorting. If the charging system is working normally then it's much more likely to have been a bad wire or switch connection in my opinion.
 
Thanks for you thoughts on this, LAB. I agree on all points. After cleaning the connections and tightening up the ignition switch, all systems now appears to be in proper order. Never a bad idea to go over the machine thoroughly anyway and it was worthwhile after finding the (potential) HT issue. On to the next issue; the source of that solitary oil drop ... .

BC
 
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An HT short circuit can only affect the HT circuit and anything that gets burnt by the sparks.

A chafed plug lead can’t make the lights go out.
 
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