Cold Start = No spark

[judge]

Greetings assembled wisdom of all things airhead.

Here's one which is foxing us currently...

My '93 R100GS-PD has developed an intermittent fault which manifests itself only when cold and only occasionally (like one in four cold starts).

Its a twin spark with twin Dyna coils and when I get the problem I get no spark on one side and the bike struggles to start. I have replaced the seemingly offending coil with a brand new one and the problem hasn't gone away (just £60 of my money).

Once the bike has started I get a good healthy spark on that side every time the bike is started again that day.

So any thoughts on where to look next?

 

[Rob Farmer]

Does it definitely not spark on one cylinder or does it only run on one cylinder when the engines cold?

I had a similar sounding problem on my ancient R100/7 that turned out to be the cork float bowl gasket. The hole for the choke circuit had gone oval letting air into the circuit which prevented the fuel be drawn through the choke system. When this happens you are effectively weakening the mixture by letting more air in making the problem worse. Once it was warm it would run fine.

If it isn't sparking - Your coils are in series so if ones working then the ignition trigger should obviously be fine. You could try swapping the coils over (just change the leads from one cylinder to the other if they are long enough) to see if the fault moves to the opposite side. Have you measured the resistances of both coil primary circuits? (That funny noise you just heard was something passing straight over your head ) You could try swapping out the apparently good original coil.

It could just be something silly like a plug breaking down. Have you tried new plugs?

Failing that I've got a couple of brand new ignition triggers sitting on the shelf as well as a Nice ignition module tester. I can post them down to you if you get stuck.

 

[AndyT]

I would try swapping things over if possible (EG HT leads) and see if the fault moves with it. Obviously only change one thing at a time or you won't know what made the difference.

Good
Luck

AndyT

 

[(RIP) Bin Ridin]

As well as the above:

Float chamber gasket has to be airtight to get the vacuum required for the rich mixture jet to work.
The said rich mixture relys on a tiny hole in the bottom of the float bowl to allow fuel to go from the float chamber into the bottom of the rich mixture chamber - can easily get blocked.
Poke gently with a copper wire strand. Shine a torch into the rich mixture chamber and look through the little hole - can you see the light?

Bin

 

[AndyT]

As well as the above:

Float chamber gasket has to be airtight to get the vacuum required for the rich mixture jet to work.
The said rich mixture relys on a tiny hole in the bottom of the float bowl to allow fuel to go from the float chamber into the bottom of the rich mixture chamber - can easily get blocked.
Poke gently with a copper wire strand. Shine a torch into the rich mixture chamber and look through the little hole - can you see the light?

Bin

Bin, From the sound of it Judges fault is electrical as he doesn't get a spark. I don't think furl or mix is an issue its something electrical. But thanks for all these tips for when my airhead starts playing up

 

[Steptoe]

Does it definitely not spark on one cylinder or does it only run on one cylinder when the engines cold?

I had a similar sounding problem on my ancient R100/7 that turned out to be the cork float bowl gasket. The hole for the choke circuit had gone oval letting air into the circuit which prevented the fuel be drawn through the choke system. When this happens you are effectively weakening the mixture by letting more air in making the problem worse. Once it was warm it would run fine.

If it isn't sparking - Your coils are in series so if ones working then the ignition trigger should obviously be fine. You could try swapping the coils over (just change the leads from one cylinder to the other if they are long enough) to see if the fault moves to the opposite side. Have you measured the resistances of both coil primary circuits? (That funny noise you just heard was something passing straight over your head ) You could try swapping out the apparently good original coil.

It could just be something silly like a plug breaking down. Have you tried new plugs?

Failing that I've got a couple of brand new ignition triggers sitting on the shelf as well as a Nice ignition module tester. I can post them down to you if you get stuck.

I would try swapping things over if possible (EG HT leads) and see if the fault moves with it. Obviously only change one thing at a time or you won't know what made the difference.

Good
Luck

AndyT

As well as the above:

Float chamber gasket has to be airtight to get the vacuum required for the rich mixture jet to work.
The said rich mixture relys on a tiny hole in the bottom of the float bowl to allow fuel to go from the float chamber into the bottom of the rich mixture chamber - can easily get blocked.
Poke gently with a copper wire strand. Shine a torch into the rich mixture chamber and look through the little hole - can you see the light?

Bin

All have been tried.

I have a beancan and ignition module to try.

 

[Phil Reynolds]

I would try treating the old "pig-iron" to a new set of HT leads. These are prone to being intermittently unreliable when they get old. Bit like human beings

 

[judge]

I would try treating the old "pig-iron" to a new set of HT leads. These are prone to being intermittently unreliable when they get old. Bit like human beings

Thanks Phil, the pig iron was treated to a new set of Magnecor HT leads less than a year ago as we did have a failure on one of them back then, HT leads all good and sound now

I'm off to St.Tippytoes later for the next instalment of Marrakesh Express, the bionic bike