Hi team, 

This time something different 89 K100 RS. Normally rides fine. Short or long distances, dry or wet weather. 
However I now had it twice that I lost the tachometer and lost power (running rough on only 2 cylinders). 
Both times this was just (minutes) after I left home with a cold engine. Given one time it was raining but this evening it was dry and cool outside.    Trying to keep the bike running, turned around, played around with the throttle, and then after a few min, it all started working as normal again.  

All clearly indicates either a failing coil or hall sensor, or potentially a loose wire/bad connection. 
While I have checked the coils before (they are ok) I have not tested the hall sensor yet. 
Will do that using this  "BertVogelK100TroubleShootingStarting.pdf" description with the Led, but my fear is that this won't tell me anything as the bike normally runs fine. (including when I parked it again).  As this only happened twice (over a 6month period), with a (reasonable) cool engine (so don't think it's heat related), I do think the more likely cause is either a loose wire. 

Where would we start? Any suggestions or learnings what connections to check first? 
- hall sensor plug to main loom
- ignition module 
- earth cables
- Tacho pickup at coil (is that a separate cable?) (need to check)
- ECU (albeit I cleaned that before, and all seems to be ok) 

Ideally I want to run the bike and then test all the connections (wiggle them around) to see if I can reproduce. But where to start? 

Any ideas?   or does anyone want to place a bet on what it is? 

Cheers,
y.

Hi Sunny,

I'll place a $1 bet on a guess. I think your thinking is good. I'd swap the coils. If the problem returns but the tacho is good, that makes the coil look bad. If the tacho is still bad, one of the other parts you mentioned is not looking good.

If you are also losing the rev counter like that my money is on the 2/3 coil defective as it also feeds the rev counter.

But it does not preclude the Hall sensor for 2/3 being bad. So swap the coils as recommended. If the problem transfers to 1/4 then its coil. If the problem remains at 2/3 then Hall sensor or wiring/connectors from it becomes prime suspect.

True but looking at the drawings makes me think it's the coil for cylinder's 1/4 that controls the tacho. So if the problem is the coil, the tacho problem won't transfer by swapping the coils, it will disappear, is what I'm thinking.

Great suggestion guys. As this happens so randomly and incidentally, I don’t want to do to many changes at the same time. Sound thinking. If it happens again, and rev counter dies, then it’s likely not the coil. Also this is the easiest thing to do, as it means I do not need to trace the hall sensor wire to the plug, to be able to test. which I assume sits under the tank.

you guys talk as if you hear (or see) from the engine if cyl 2&3 or 1&4 fail? Or is there another trick besides pulling spark plugs to find out what cylinder (and coil) are not firing?
(as this happens on the road, and so randomly) and the engine dies if I do not keep reving it by hand, removing spark plug cover and pull plugs isn’t really handy or quick)

(Got to love these challenges with old bikes !)

A timing gun clamp around spark plug 1 lead, for example, is a quick way to see if it's getting spark.

daveyson wrote:True but looking at the drawings makes me think it's the coil for cylinder's 1/4 that controls the tacho. So if the problem is the coil, the tacho problem won't transfer by swapping the coils, it will disappear, is what I'm thinking.

Thats good thinking and at no cost.

Don't dismiss the idea of a bad plug, HT lead or plug bobbins!!

daveyson wrote:A timing gun clamp around spark plug 1 lead, for example, is a quick way to see if it's getting spark.

Very true, but… I don’t usually carry that with me on the road. And this only happens very very occasionally . Will put in some tools that allow me to pull leads when this happens again.