Hey guys

I have a problem with my 86 K100Rt and after reading a few threads here it sounds like it could be many many reasons for this happening. But first... the symptoms....

problem in detail:
this regularly occurs at starting
often i will try to start the bike and when i hit the start button all the ignition lights will turn dull (kinda like a flat battery) when i try to start it the fuel pump will start and i might get 1 turn from the starter or nothing at all (it depends)... if i flick the kill switch and set it back to normal (do you call this cycling?) it might start but often I need to do this 4 or 5 times or more until i can get the starter to engage long enough to get the bike started. if i don't flick the kill switch again after a failed start the starter will not engage at all when i hit the start button (only the fuel pump will start but nothing from the starter).

also the following has happened a few times which is a worry... if i am in traffic i would hate for it to happen so gotta get this sorted ASAP
Yesterday i was cruising in 3rd gear on a quiet suburban street after grabbing some supplies and the bike stopped for no apparent reason. It has happened a couple of times before on gear changes and i was unsure if it actually happened or not if you know what i mean... when i came to a stop I hit the start button and the starter cranked hard but the engine would not fire.. didnt have tools on me to pull out a plug so can't tell you any if they were wet or not.

this weekend i will pull the tank off and check the temp sensor and earths and all connections that i can find. I am thinking a bad earth may be the most likely suspect but will wait to see what we find. I also think this may be 2 separate problems

if anyone else has had the same experience feel free to let me know, otherwise i will update you as to what i find.

cheers
viney

Hi Viney,
There are two common problems with all makes of bikes of our vintage, bad power supply contacts and bad earths all caused by the life they lead with all their bits in the open.

A good clean up of every contact on every bulb, plug, socket and relay base with DeoxIt and a thorough clean up of the bundle of earths under the tank and down by the footrest and onto the battery/starter/relay box. Pay attention to the fuel tank connector, that is prone to contact problems as it is exposed and a good connection here is vital to run the fuel pump, again the DeoxIt treatment and making sure the socket grips the pins.

There is only one BMW specific thing that I can think of that is also a regular problem and that is the earthing through the starter motor brushes. BMW engineers chose to earth quite a lot of stuff effectively through the starter motor commutator via a relay and wear or sticking of the brushes causes a lot of very odd problems. As your bike is an ex-Police bike it is likely to be high mileage and hence a refurbishment of the starter is probably overdue, not too hard to do just a little fiddly to get out and then have at it. Get spare brushes before you start.
Regards,
K-BIKE

Hi viney the one common thread I can see in your problem is the clutch or more specifically the clutch switch which is a crucial part of the starting circuit.

It is the same switch as the brake light switches and not uncommon for problems so worth checking.

The other thing specific other than K-Bike's advice is the ignition switch contacts. On mine they were bad enough to be measurable with a multi-meter and I had to file mine slightly to improve the result.

Let us know how you get on,

Regards,

Phil

Hey guys thanks for your input.

OK here is an update from today

Checked all main connections eg. starter, alternator, battery, earths etc. even went over the coils and computer box under the seat (behind the fuse box) to make sure all was fine there. all connections seemed fine but gave them all a touch up anyway since i had her opened up.

The earth under the tank took a bit to find since i am fairly tall i couldn't see it under the framework til i got down on one knee. I wasn't convinced that a bad earth was the cause as it all seemed fine so i pulled out the ignition barrel... after a bit of trial and error trying not to destroy it i got it apart and and there is was.... one of the contacts on the "Y" section of the ignition switch was all brown and coated with some tough gunk I am amazed it ran at all. the rest of the contacts were almost perfect but went over the entire thing with some 1200 grit sandpaper to make sure

Just finished putting the ignition back together and its now 3am so i don't have time to test it but will put her back together tomorrow

will let you guys know when it is back together

put the bike back together today.... yep it was the ignition switch contacts
thought while i had it apart i would whack a kill switch on it for a bit of extra security which only took a few minutes but

Thanks for your advice guys if it wasn't for this forum it could have taken weeks or months to get to the source

Well Done Viney,
Great news and cleaning up all the contacts and earths is a major for preventing future problems.
Regards,
K-BIKE