Ignition cuts out shortly after getting under way

Strangest thing happened to me yesterday. I took my 850 mk2 out yesterday, and after traveling about 4 miles, the engine quit. There was no sputtering. It was as if the ignition key was shut off. Another hot day here in New England, about 86 deg. and humid, but I was doing about 50mph with no traffic, so I think airflow should have been sufficient. The bike certainly was not acting like it was too hot, which I’m pretty familiar with. Nevertheless, I waited 20 min. or so and tried to kick it over but nothing. I took the tank off checked all connections, put it back on, tried again and I started easily. I motored off, didn’t get more than a half mile and it shut off again. Fortunately this time it was right in front of a fellow who transports motorcycles for a living! So I got the bike home to troubleshoot and couldn’t find anything obvious. The bike continues to do the same thing. Runs for a minute and dies. Wait 15-20min and the same thing. It has a Lucas 3phase alt and Boyer ignition. Otherwise ignition and charging system is stock. Any ideas? Many thanks!

Semitone said:

Strangest thing happened to me yesterday. I took my 850 mk2 out yesterday, and after traveling about 4 miles, the engine quit. There was no sputtering. It was as if the ignition key was shut off. Another hot day here in New England, about 86 deg. and humid, but I was doing about 50mph with no traffic, so I think airflow should have been sufficient. The bike certainly was not acting like it was too hot, which I’m pretty familiar with. Nevertheless, I waited 20 min. or so and tried to kick it over but nothing. I took the tank off checked all connections, put it back on, tried again and I started easily. I motored off, didn’t get more than a half mile and it shut off again. Fortunately this time it was right in front of a fellow who transports motorcycles for a living! So I got the bike home to troubleshoot and couldn’t find anything obvious. The bike continues to do the same thing. Runs for a minute and dies. Wait 15-20min and the same thing. It has a Lucas 3phase alt and Boyer ignition. Otherwise ignition and charging system is stock. Any ideas? Many thanks!

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Maybe your fuel tank is not venting properly. Something to check.

I had issues like that on my 850 mk2 but not as frequent. More random. The issues went away when I replaced the electrical connector block under the fuel tank.

Battery may be on the way out. Builds up enough of a charge while sitting to start but not enough to keep running.

The key stop working on mine.

Sounds exactly like the famous Boyer ignition cut off.

Check the wires down near the trigger on the end of the camshaft. The wire breaks inside the plastic sheath. Check very carefully giving them a good pull. It can be hard to spot.

Very commonly the bike will cut out after a few km and will start again after 30 mins or so when it has cooled down and the wire contact is restored.

If it’s Boyer Micro Power ignition with a blue box, RF interference from the HT (plugs) circuit can cause sudden shutdown, but it would usually start again immediately after switching off and on again.

I remove the troublesome wire tails from the stator plate, and fit a screw terminal block, as seen here (this is still available if anyone is interested)

Check your coil resistances, mine would cut-off with total coil resistance of 2.9ohms as the box overheated. 15 mins later it would start and run for 10 mins, rinse and repeat. Cured by replacing coil with 3.1 ohms resistance.

You can temporary hotwire the ignition system for troubleshooting.
This eliminates bad wiring connectors, ignition key, and kill switch.

Test.
Don't guess.

take a test lead, power the boyer directly from the battery.

Test ride.

Report back.

concours said:

Test.
Don't guess.

take a test lead, power the boyer directly from the battery.

Test ride.

Report back.

Click to expand...

^^^^^^ this is the method to live by,...... however, once you've tested all the things that can be tested you should understand that intermittent problems can defy testing since they may only break down under actual running situations.

Things like ignition switches actually can corrode inside and cause the contacts to be weak so they heat up and break down once they get too hot. I have drilled the rivets out of a sealed ignition switch to freshen it up and remove corrosion then rivet it back together when my key switch got old and crusty.

another possible cause is some sort of ground fault contact inside the headlight shell. Usually there's a lot of connections made inside the shell. If they ground against each other or the shell, it can act like a kill switch. I've had that happen too and some strategically placed electrical tape and shrink tubing can usually eliminate those issues...

Another cause is the handlebar clusters pinching a wire in the control harness against the metal handlebar. Also a corroded kill switch on the clusters is a common cause.....

Electrical issues take patience and a methodical approach... I like to replace the battery with my tester leads on the continuity setting where it makes an annoying tone, then go around wiggling the harness and components to see if you can find the poorly conducting component or the ground fault.

You'll find the issue, just be methodical...

johnm said:

Sounds exactly like the famous Boyer ignition cut off.

Check the wires down near the trigger on the end of the camshaft. The wire breaks inside the plastic sheath. Check very carefully giving them a good pull. It can be hard to spot.

Very commonly the bike will cut out after a few km and will start again after 30 mins or so when it has cooled down and the wire contact is restored.

Click to expand...

I've found pulling and wiggling those wires while engine idling allowed me to recreate the intermittent failure

I had an intermittant fuse blowing. Ultimately found the new multi connector block under tank had a tear between connectors on the inside....looked totally fine from out side but when those connectors made contact, poof went the fuse. Now I just use a few double & triple connectors there, no multi block...and everything fits better under thank as a side effect.

Another item to suspect...the kill switch contacts might be corroded....give them a touch of emery paper.

Have a load test of you battery performed. I suspect it doesn't hold its charge and the voltage drops too much, which will cause the Boyer EI box to quit (depending on which version).
When left with no current draw, a damaged battery may regain some of its charge, which will quickly disappear when loaded again. This scenario seems to coincide with your experience.

- Knut

Agree with Knut. Before doing any electrical system diagnosis you need to start with a known good battery.

Hi Semitone,
Do you have a “kill” switch ?

Thanks for all the input guys. So I failed to mention that I did a full restoration on this bike -finished about a month ago. It has new Amal carbs, gaskets and gas cap. I know this not a fuel delivery problem by the way the engine cuts out. I refurbished the wiring harness, replacing a lot of the connections. The ignition switch was also replaced. The battery is probably about six months old, and charging system is work well. I will do a load test nevertheless. The Boyer Branson ignition however is about 20 years old. I have not checked the connections inside the headlamp shell. I will do this next. I also like the idea of home run wire from battery to BB unit. Actually, all great suggestions about the unit. I actually have an extra that I will swap in. More to come!

TBW said:

Hi Semitone,
Do you have a “kill” switch ?

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I do have a kill switch. I see where your going. Will check contacts

Tornado said:

I had an intermittant fuse blowing. Ultimately found the new multi connector block under tank had a tear between connectors on the inside....looked totally fine from out side but when those connectors made contact, poof went the fuse. Now I just use a few double & triple connectors there, no multi block...and everything fits better under thank as a side effect.

Another item to suspect...the kill switch contacts might be corroded....give them a touch of emery paper.

Click to expand...

I’m not blowing any fuses. In fact all other electrical elements work. It’s just the ignition that cuts out and returns 15-20min later. TBW also mentioned kill switch.