#0018458 (K100RS, EUR, 05/1984, code 0503) has taught me a lot about air leaks in the intake, and how beastly a beauty can be. it's in denmark, and i'll be visiting it in mid september, and i've set aside a day of repair before i ride down into germany. the previous owner did a wonderful job on the paint and powder coating but appears to have pulled and reinserted the engine and drive train without taking it apart. so, we've gone through it extensively, including all new rubber in the intake, all new vacuum hoses, new plugs and wires, spline lube, all fluids. this bike has taken me across sweden into estonia and down through latvia and lithuania and then back through sweden; it's also made the reverse loop but also including finland. it's pretty solid. you can see from the picture below that i put LT bars on it. you can't see from the picture that the damned mirrors keep falling off and so one fix i'll be doing shortly is to put stock K100 (short) mirrors on it, brush guards with integrated turn signals, and then i'll remove the mirror/signal ears and take it to a body shop for fill and paint. (this is a change i've enjoyed on the similar bike i keep at home in california.)

anyway there are two tuning/running issues that i'd like to diagnose and fix:

1. takes ten to 20 minutes to start up, even in shirt-sleeve weather. i am always the last biker off the ferry because i have to start it a dozen times and feather the throttle until it's hot enough to start the fan running -- otherwise it dies as soon as i apply power.

2. dies when returning to idle after running at speed, like when coasting to a stop sign. even if i downshift and apply engine braking in every gear on the way down, it will die when i clutch for the stop. i usually then pop the clutch to make it start again, and feather the throttle until i can start up again.

by now i have replaced the coolant temperature sensor, cleaned and tested every harness connection, cleaned and tested the idle switch, cleaned and tested the enrichener ("choke") mechanism, since this is a 280Z-era bosch jetronic system and i know its weaknesses well. also replaced the vapor recovery hose, and every bit of rubber and plastic in the intake system. and of course i've acquired the necessary spare AFM and ECU, and have cleaned those connectors as well. i used to have worse problems with this bike! but i still have two.

astonishingly, i have not checked the ignition timing, hall sensors, or fuel pressure. so, those are next.

however, if my symptoms sound familiar, and you have additional advice, you will find me an avid listener.

If your HT lines are original, it could be a wise idea to change them.
Which spark plugs do you use ?  Are their electrodes all of the same colour? Which colour?
When is the last time you achieved a serious throttles synchro? By serious I mean making sure that the trottles are all well closed when you tell the Jetronic that they are. It is not because de depressions are perfectly balanced on all 4 cylindres that the throttles synchro is good. If you lie about the starting point, all the calculations are wrong.

Poupy wrote:If your HT lines are original, it could be a wise idea to change them.
Which spark plugs do you use ?  Are their electrodes all of the same colour? Which colour?
When is the last time you achieved a serious throttles synchro? By serious I mean making sure that the trottles are all well closed when you tell the Jetronic that they are. It is not because de depressions are perfectly balanced on all 4 cylindres that the throttles synchro is good. If you lie about the starting point, all the calculations are wrong.

every fuel, vacuum, intake, and ignition-wire line on the bike has been replaced without any change to these symptoms.

i use bosch X5DC plugs, which have been replaced without changing my symptoms. i'll check for color when i get back to denmark in ~two weeks. another responder advised me to check them for wet-vs-dry during the hard-to-start interval, which seems obvious now that he said that, but wasn't to me.

i paid a factory trained dealer tech (in 0lgod) to re-synch the throttle bodies, since i lacked the tools and confidence to do it myself. this improved some other symptoms but did not change the hard starting or dies-at-stop-signs symptoms.

thank you for your answer, i will report back any findings or resolutions.

mibh wrote: i will report back any findings or resolutions.

If you haven't done a compression check, and a valve clearance check, you should get right on those fundamentals and not dawdle. I don't read that you have.

Check the coils as described in this:.

http://www.kforum-tech.com/electrical/EFI/bike-wont-start-EN.htm

Laitch wrote:

mibh wrote: i will report back any findings or resolutions.

If you haven't done a compression check, and a valve clearance check, you should get right on those fundamentals and not dawdle. I don't read that you have.

+1

Laitch wrote:

mibh wrote: i will report back any findings or resolutions.

If you haven't done a compression check, and a valve clearance check, you should get right on those fundamentals and not dawdle. I don't read that you have.

you could have read that i have not, and that these things are already on the list. what i was perhaps less clear about is that my travel schedule will give me one repair day with this motorcycle in westernmost denmark before i ride it down into germany. whatever tools i may need i have to acquire locally (so far it's a compression tester and a fuel pressure tester). whatever hypotheses i want to test i have to know before hand. so that's why i'm asking for your wisdom rather than walking over and performing each test as i learn them. i deeply thank all of you for helping prepare me for this trip. you guys are great.

duck wrote:Check the coils as described in this:.

http://www.kforum-tech.com/electrical/EFI/bike-wont-start-EN.htm

ty, ty, ty! this shall be done.

mibh wrote:you could have read that i have not, and that these things are already on the list. what i was perhaps less clear about is that my travel schedule will . . .

Clearly you are busy and your presence is in great demand so instead of distracting you with tiresome advice, I'll just start carbohydrate-loading early in preparation for the dance tent down here—the destination for Sunday's ride. Maybe I'll see you there. Look for the guy in a clerical collar carrying a basket of emojis.  

Adiós.