R100RS cut out on me and won't start

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R100RS cut out on me and won't start

Postby Glynd3 » Fri Aug 25, 2017 10:17 am

Good morning
1981 R100RS
I was riding my bike last night and all of a sudden I heard a little clanging noise, a bit like a stone had come up off the road and hit the bike (No front mudguard fitted) then my rev counter started to fluctuate and the engine started running a little rough. I came to the next junction, indicated, but the indicators didn't work. I carried on for a mile or so, but at the next junction, she cut out and then wouldn't restart. Has anyone else experienced anything like this? I would welcome any advice on helping me fix the problem, please.

Just a note, the battery is fully charged and the lights work, when I press the starter I can hear a low electrical clicking sound. Hope that helps.

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Re: R100RS cut out on me and won't start

Postby Galactic Greyhound » Fri Aug 25, 2017 1:16 pm

Hi Glyn,

Does the Horn work?

When you say that the bike will not restart, do you mean it cranks over but will not fire up or that you only hear a clicking sound?
If only a clicking sound, is it a single click or a series of clicks like a machine gun?

As a preliminary to any further diagnostic/faulting work, could you disconnect your Battery and clean the Battery terminals and the cable terminals to bright metal.

Then, before re-connecting, LIGHTLY smear the Battery and cable terminals with Vaseline and then reconnect.

Also check at the Engine/Gearbox Main Earth connection from the Battery Negative (earth) terminal that the Main Earth connection is sound.
Be VERY careful if disconnecting this for cleaning as above as, if it utilises the Gearbox Speedo Cable bolt on the gearbox casing, then this is a hollow vented bolt which can shear or be seized in the Gearbox threads.

Next, see if the bike will start.

If not, despite you saying the Battery is fully charged, put the Battery on charge overnight for 12 hours and then try starting again.

We can take it further from there if necessary. :smile:

R1100RT 1996.
Sent by Boson Quantum Transmission from the Starship 'Galahad'.
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Re: R100RS cut out on me and won't start

Postby george baker » Wed Nov 22, 2017 12:11 pm

Hi Glynd3
any feedback? Ced kindly provided advice and I am sure he would like to know if his solution worked for you

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Might help...

Postby SteveD » Mon Dec 11, 2017 11:46 pm

This should be in Airheads. :-k I'm thinking coil or cracked left coil mount (earth point). An initial eyeball for the obvious possibilities then if no success there move to a systematic review.

Troubleshooting BMW-fitted electronic ignition Airheads 1981-1996

* Disconnect battery neg from battery before removal of the front engine cover.

Disconnect bean can 3-pin connector below diode board. It has a wire bail that is a PITA to remove.
Use a hooked pick to pull one end of the clip and the connector will release. Handle the connector carefully, old ones often crumble in your hands. If that starts to happen, MARK each of the three wires as to position, and make a note of the marks. You can insulate the three wires and connect them individually as a makeshift.

Position a wire or straightened, unpainted paper clip into the centre female terminal on the harness side of the connection.

Remove ONE spark plug. Reconnect the plug to the HT cable, firmly ground the spark plug in a location where it cannot fall off, has good ground and where you can clearly see the spark. Best in low light.

Reconnect battery, turn on IGN switch, kill switch ON. Instrument lamps ON.

COIL and WIRING TEST: Flick kill switch on and off as you watch the spark plug. Got ONE spark each time? The coils are OK, stop suspecting them.

BEAN CAN TEST: Scratch the paper clip across the alternator housing (unpainted aluminium=good ground) like striking a match, while looking at the spark plug:

IF you see spark as you scratch the paper clip across ground, the BEAN CAN is bad - probably failed HES. Repair or replace bean can. BMW doesn't sell them anymore.

If you DO NOT SEE spark, most likely culprit is the Ignition Control Unit under the tank. (By process of elimination.)

If you remove or replace the ICU, clean off the mount, remove and replace the old heat sink compound. Don’t install without heat sink compound. Replace it every few years. More if you ride a lot in hot conditions.

• Don’t forget to disconnect the battery before reinstalling the front cover.

Paraphrased from a Tom Cutter article.
Cheers, Steve.
1982 R100RS, 2006 K1200R.
Stay horizontal!


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