Author Topic: early K100RS hard start, dies at stop signs  (Read 293 times)

Offline mibh

  • Motobrick Curious
  • Posts: 20
  • BMW rider since 1984
early K100RS hard start, dies at stop signs
« on: August 24, 2019, 07:44:08 PM »
#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.

  • Skanderborg, DK; California, US; London, UK
  • R100 '81, '84, K100RS '85 (US), K100RS '85 (DK), K100RS '84 (UK)

Offline E30_Crazy

  • ^ Motobrick Curious
  • Posts: 88
Re: early K100RS hard start, dies at stop signs
« Reply #1 on: August 24, 2019, 11:17:45 PM »
Besides what you already mentioned at the end, all that I can think of would be valve clearances, throttlebody vacuum sync, and compression test.
  • Newport News, Virginia
  • '85 K100, '93 K1100RS

Offline Martin

  • ^ Quintessential Motobricker
  • Posts: 3859
Re: early K100RS hard start, dies at stop signs
« Reply #2 on: August 25, 2019, 05:04:26 AM »
After a few attempts to start have failed pull the spark plugs. Are they wet or dry.
Regards Martin.
  • North Lakes Queensland Australia
  • 1992 K75s Hybrid, Lefaux, Vespa V twin.

Offline alabrew

  • ^ Proficient Motobricker
  • Posts: 132
Re: early K100RS hard start, dies at stop signs
« Reply #3 on: August 25, 2019, 11:28:12 AM »
My '85's hard starting problem was traced down to the air temperature meter in the AFM was broken and telling the computer it was 158 degrees.
Always ran better on the way home on those 98 degree days.
  • Alabama
  • 1985 K100, 1991 K100RS
Also:
2005 K1200LT
1979 R65
200,000 miles on BMW motorcycles

Offline billday

  • ^ Quintessential Motobricker
  • Posts: 1160
Re: early K100RS hard start, dies at stop signs
« Reply #4 on: August 25, 2019, 02:40:14 PM »
Are you comparison shopping for free advice? If so let us know the results of your research.

http://www.k100-forum.com/t15369-early-k100rs-hard-start-dies-at-stop-signs?_branch_match_id=686188925072582930
  • New York State, USA 10977
  • 1985 K100

Offline mibh

  • Motobrick Curious
  • Posts: 20
  • BMW rider since 1984
Re: early K100RS hard start, dies at stop signs
« Reply #5 on: August 26, 2019, 02:32:20 PM »
Besides what you already mentioned at the end, all that I can think of would be valve clearances, throttlebody vacuum sync, and compression test.

ty. will bring a valve cover gasket with me, and a collection of shims. tb sync was already done to fix a different earlier problem. before i pack a compression tester can you tell me the theory by which that could produce the observed effects?
  • Skanderborg, DK; California, US; London, UK
  • R100 '81, '84, K100RS '85 (US), K100RS '85 (DK), K100RS '84 (UK)

Offline mibh

  • Motobrick Curious
  • Posts: 20
  • BMW rider since 1984
Re: early K100RS hard start, dies at stop signs
« Reply #6 on: August 26, 2019, 02:34:36 PM »
After a few attempts to start have failed pull the spark plugs. Are they wet or dry.
Regards Martin.

that's a fantastically useful question -- i don't know, because eventually, it does start. but i have to crank at half-open throttle for five to ten seconds to make it do so, and then i have to feather the throttle to keep it alive. i will try cranking it cold with only the enrichener ("choke") and no throttle and then check the plugs as you suggest.
  • Skanderborg, DK; California, US; London, UK
  • R100 '81, '84, K100RS '85 (US), K100RS '85 (DK), K100RS '84 (UK)

Offline mibh

  • Motobrick Curious
  • Posts: 20
  • BMW rider since 1984
Re: early K100RS hard start, dies at stop signs
« Reply #7 on: August 26, 2019, 02:37:24 PM »
My '85's hard starting problem was traced down to the air temperature meter in the AFM was broken and telling the computer it was 158 degrees.
Always ran better on the way home on those 98 degree days.

since it's possible that my original and replacement AFM are both broken in the exact same way, i will ohm them out per your implication. i wish the FSM would tell me what resistance to expect from the harness, in case there's a broken connection upstream of the part i can see and clean. i guess i'll pull the ECU harness and test the AFM temperature from there. ty!
  • Skanderborg, DK; California, US; London, UK
  • R100 '81, '84, K100RS '85 (US), K100RS '85 (DK), K100RS '84 (UK)

Offline mibh

  • Motobrick Curious
  • Posts: 20
  • BMW rider since 1984
Re: early K100RS hard start, dies at stop signs
« Reply #8 on: August 26, 2019, 02:39:15 PM »
Are you comparison shopping for free advice? If so let us know the results of your research.

whatever useful thing i learn anywhere, i will share everywhere. of the various brick-related forums, this one (motobrick.com) has been the best so far.
  • Skanderborg, DK; California, US; London, UK
  • R100 '81, '84, K100RS '85 (US), K100RS '85 (DK), K100RS '84 (UK)

Offline johnny

  • TrailBrakingThrottleWhacker
  • ^ Quintessential Motobricker
  • Posts: 6758
  • Whacking...n...Chopping Sliding...n...High Siding
Re: early K100RS hard start, dies at stop signs
« Reply #9 on: August 26, 2019, 03:10:32 PM »
 :xenogreetings...

its the fore pin... dissenters abide...


* Screenshot_20190826-144455.jpg (17.62 kB . 393x576 - viewed 38 times)


* Screenshot_20190826-144536.jpg (27.9 kB . 492x576 - viewed 37 times)

j o
  • :johnny i parks my 96 eleven hundert rs motobrick in dodge county cheezconsin  :johnny

Offline E30_Crazy

  • ^ Motobrick Curious
  • Posts: 88
Re: early K100RS hard start, dies at stop signs
« Reply #10 on: August 27, 2019, 01:11:55 AM »
ty. will bring a valve cover gasket with me, and a collection of shims. tb sync was already done to fix a different earlier problem. before i pack a compression tester can you tell me the theory by which that could produce the observed effects?

You have basically covered the electrical and wiring side of the house, so assuming nothing was missed/mis-read, or changed out for another bad part, all you have left are the basic principles an internal combustion engine - Suck, squeeze, bang, blow.

- You covered the "suck" part with the TB sync, plastics and rubbers (and 1/3 of "bang," technically)

- You stated your intentions of checking the pump/fuel pressure side of things, this is 2/3 of "bang"

- The bike is said to be an '84, so you won't have an 02 sensor. Unless you have a rats/birds nest that won't dislodge itself or burn out, I would say "blow" is all good.

- So, left with "squeeze," is compression (valves, their adjustment, head gasket, and piston rings). Where a compression test would be a quick identifier to see if there could be a problem with any one of these, all at once. If you have good compression, it would make little sense to individually chase valves (other than verifying clearances) rings, or head gasket.


Maybe one thing not mentioned yet, unless you've already tested, would be the coils (along with timing/hall sensor, the remaining 1/3 of "bang"). Early Ks are known for coils to fail, and even "newer" Ks with the updated coils are getting on in age now. The Ignition Control Unit could also be considered, but rough running is more a sign of a failing/weak coil, where a bad ICU would, in my mind, make for a non-starter, or full shut-down after the electrics inside decide to quit (heat, current, overload, resistance, etc)
  • Newport News, Virginia
  • '85 K100, '93 K1100RS