My recently acquired 1986 K100 ‘project’ has been giving me the runaround since we met.  It seems every time I get her booked in for an MoT she refuses to start, despite running well the day before. 

Finally managed to get down to the test station yesterday for the old girl’s first MoT in three years and very proud to say (and relieved!) that she passed with flying colours!  So now I know she’s a keeper I want to understand this starting issue.  I finally got the bike going yesterday after I disconnected/reconnected the connector block on the black cable which connects into the computer and is routed down the RHS front fork leg. The symptoms are that the pump delivers good pressure to the fuel rail but the plugs are still dry after cranking the motor. There is a  healthy looking spark at all four  cylinders too. I found the link below on this forum and the troubleshooting used seems relevant to the issue I have, but it’s not clear how this ever got resolved. 

The bike has restarted OK since yesterday, but having not really identified a root cause for the problem I’m not confident I won’t get stranded somewhere in the future.  

Just wondering if anyone on this fantastic group has any strong views or pearls of wisdom regarding this apparently random starting issue?
My recently acquired 1986 K100 ‘project’ has been giving me the runaround since we met.  It seems every time I get her booked in for an MoT she refuses to start, despite running well the day before. 

Finally managed to get down to the test station yesterday for the old girl’s first MoT in three years and very proud to say (and relieved!) that she passed with flying colours!  So now I know she’s a keeper I want to understand this starting issue.  I finally got the bike going yesterday after I disconnected/reconnected the connector block on the black cable which connects into the computer and is routed down the RHS front fork leg. The symptoms are that the pump delivers good pressure to the fuel rail but the plugs are still dry after cranking the motor. There is a  healthy looking spark at all four  cylinders too. I found the link below on this forum and the troubleshooting used seems relevant to the issue I have, but it’s not clear how this ever got resolved. 

https://www.k100-forum.com/t1686-fuel-pump-not-powering-up?fbclid=IwAR2K6SOvpbFHVciZEHEbMgmoiZnGyf0jyay9UglkqWzGHNPSnU_-V1XBJz4_aem_AUUNGuY27S97Ct7xAOH3NPv9yAFCdUhFZtWbB3PQcYWHHepiCwKV45gRQ-ovd8s_3Fc#lupxuziujoeuna35ro

The bike has restarted OK since yesterday, but having not really identified a root cause for the problem I’m not confident I won’t get stranded somewhere in the future.  
 
Any thoughts please?

Remove the connector for the L-Jetronic under the seat and inspect the terminals for corrosion.

Sometimes just disconnecting and reseating that connector solidly helps.

Thank you, I shall investigate this.

“ I disconnected/reconnected the connector block on the black cable which connects into the computer and is routed down the RHS front fork leg”

Not sure what cable this description refers to - the routed down the rhs fork is a bit confusing…maybe you are talking about the top section of the fork only? The only wire that routes down my rhs fork is for the abs sensor…and judging by your photo in your introduction post, you don’t have abs.


A lot of intermittent problems are caused by dirty/loose earth connections. Definitely worth your time (and peace of mind) to get a can of electrical contact cleaner (deoxit is best but pricey and seems to only available on line in the UK) and give the all the connectors and good squirt and disconnect & clean the main earth point (bolt on centre frame under the fuel tank on left hand side) and the earth connection to the gearbox housing (left hand side below and just behind the coil cover). 

Two thoughts.

1
Have you ever noticed that bricks are like horses? If you fear something (MoT) they can sense it, and won't start, or try to buck you off. Never fear (Have you ever noticed that my avatar has two iron horses? One is made with horse shoes)

2
An intermittent problem is the most annoying problem you could have. And they wonder why a bloke grabs a gun, goes down to Macca's, and starts shooting people. But better to make it a constant problem, then fix it.

Fuel at the fuel rail, but dry cylinders makes you think the injectors aren't opening. If the injectors aren't getting power, they won't open. If they're not getting earth, they won't open. Wiggle the loom from the injectors here and there to see if that reproduces the problem, or eliminates it. I'll guess they're not getting earth, but testing with a computer safe test light will reveal what's missing.

duck wrote:Remove the connector for the L-Jetronic under the seat and inspect the terminals for corrosion.

Sometimes just disconnecting and reseating that connector solidly helps.

At one point Red was stopping intermittently on the freeway. I found that disconnecting and reconnecting the connector, reasonably easily done as these things go, would get the bike restarted. So thinking it was obviously electrical I fell back on a crude fix. I took a needle nose pliars and rotated each of the spade connectors in the  L-Jetronic box by 3-4 degrees. Just barely twisting them. This  ended the problem till I got back home from that ride and I could go in and clean them all. And that eliminated the problem. Well I've not had it come back in the same way.

Ignition Control Unit is a possible cause, if you have not identified anything else, My intermittent fault was down to a faulty unit, good luck and enjoy your K.