Intermitent ignition failure BMW r80

[Falconer]

Cruising along when it will backfire and cut out. Stop for a while and it will start as good as new most times. Have replaced coil, cleaned switches, checked connections. What fixes it is if you unplug and replug the coil? Have replaced these plugs with no change. I hate intermittent problems, any suggestions?

[Brian d marge]

what year R80 and what type of Ignition ? ...

A loose connection around the coil ? so when you unplug it you move a wire , restoring the connection again

Just a thought need more info on bike

Stephen

[Falconer]

1986 BMW r80; electronic ignition; Not sure what gives in the engine housing but the wires come out of the engine, into a matchbox size electronic looking thing that has a heat sink of a similar size on it. fully sealed, not even a mould mark on it. The wires go into the loom and then go to the coil that is a twin spark giving of a wasted spark to one cylinder each fire. Hope this helps.

[Brian d marge]

grasping at straws here because I cant find any wiring diag / info on the net and my memory is shocking

heres what I think

The backfiring in the spark happening at the wrong time , So either the Ignition unit is recieving a signal at the wrong time or it detonation

We can rule out detonation as it runs fine usually , just suddenly bang and stops

So the ignition unit is recieving a signal , So the pick ups in the engine are in good cond, ? and the wires going to the loom to the ignition box are good in condition

If it is a scr ignition unit then the gate of the scr will be recieving a signal , either through incorrectly timed pick ups , ( shorting leads of pick ups,,across terminal pins ...?? renew dodgy looking connectors and clean up earths ... ) , or the wire that comes from the pick ups to trigger the gate will be shorting ( cant remember if the gate is grounded or positive )

Which means the scr is recieving a signal telling it to release battery voltage which goes about its usual business of making a spark

The coil is fine , ( you have replaced it and it still does it , So the only thing that is happening is that as you replace the coil you are moving wires .....the broken shorting wire could be there ??

Finally if you blow fuses use a winker relay across the fuse , it will cut in and out allowing the fault/wires to be fiddled with to track it down , rather than keep blowing fuses


Sorry for the garbled reply , With out any info I am stabbing in the dark...

all the books I have here end about 1974 ......

Stephen

Let us know what you find

[Falconer]

Thank for those ideas. Is the SCR in the little box with the heat sink that is seeled? I think the back fire is just unburnt gas after the ignition failure, set of by a random spark, because the motor just dies after this. It has all the hall marks of a dry joint, ( not the cannibis type). Is it just a hall effect points in the engine housing and should I be looking in there?

[Brian d marge]

Yes they are just Hall effect from what I remember ( please someone jump in as my memory is shocking )

Anyway the nearest wiring diagram I can find is for a Gs, and no mention of the little black box

Here is a the ignition circiut for that , is it the same ??

For a spark to occur, the primary current has to collapse, or be suddenly generated flow though the primary windings , My guess would be a broken or chafed black wire or a bad earth causing it to leak to another path .

What bout the rev counter , I have had bike cause trouble when the rev counter has / is packing up , try running with oput a rev counter and see if the problem occurs

If I can find a proper manual , I could be of more assistance , as it was nearly 8 years ago since I last worked on BMWs

Stephen

[Falconer]

For your very detailed circuit diag and your thoughts. It must have taken you time and I do appreciate it. Knowing the parts involved I had a good look at them and cleaned all the plugs and sockets, took it for a ride and all seems good. But it has done this before but if it aint broke just now I can't fix it. I only have a few more parts to replace so I should have it under control if it happens again.
Thanks once again for your efforts.

[davereid]

I had a similar problem with a GPz600. It would run like a dream, then just backfire and stop. Pull over, look for a problem - would never find one, it would just restart and away you go.

I bypassed the kill switch, I tried hotwiring the IGN module direct to the battery in case it was a faulty ign switch etc etc. But it would never fail when I had the chance to look for a fault. Late for work it would stop. Plenty of time on a saturday it would go.

Good luck !

[Brian d marge]

On yamaha FZ400 s they can run cold , and foul a cylinder, had it happen twice on separate bikes, still dont know the answer you name it it was with in spec , but a short time later ,,they would foul a cylinder


Stephen

[Bonez]

Could be the pulser unit breaking down when hot.
The bit under the front engine cover that replaced good ol points. Old man had to replace his some years ago.

Also happened on my brothers ol CB650Z, one spair of cylinders would stop firing. Stop for 10 minutes or so and away they went again.

[Brian d marge]

Could be the pulser unit breaking down when hot.
The bit under the front engine cover that replaced good ol points. Old man had to replace his some years ago.

Also happened on my brothers ol CB650Z, one spair of cylinders would stop firing. Stop for 10 minutes or so and away they went again.

I called them pick ups . but hall effect . pulsar units .. and good to see someone else has had trouble with them , The insulation on bikes of that era would be starting to cause probs, and the Jap ones were ,,,err cheap , bad luck if you had a cx 500 ,,, engine out for alternator ,,all because of a wrong sized connector !

even worse luck if it was a turbo, and the turbo bolts are siezed

Keep us informed of what you find !!

good luck

Stephen