Author Topic: 1985 BMW K100 random miss  (Read 20965 times)

Offline The Mighty Gryphon

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Re: 1985 BMW K100 random miss
« Reply #75 on: January 06, 2019, 09:35:12 AM »
Congratulations!  Rough idle with cap removed indicates that you have successfully compensated for the intake air leak at idle.

Your jiggling of the MAF wiper has successfully enriched the idle mixture to keep up with whatever intake leaks you had.  As the law of unintended consequences demands, you have also enriched the mixture over the entire range of throttle settings.  Do you think that might be reducing the thermal efficiency of the engine?   All my bikes get between 45 and 48 mpg.
  • In my garage in Marilla, NY
  • '91K100RS White/Blue
Current:
'91 K100RS16V "Moby Brick Too"

Past:
'94 K75RT "Ilsa, She Wolf of the SS"
'92 K100RS16V "Moby Brick" (RIP, deceased in a vehicular assault)
'94 K75S Special Edition Dakar Yellow "Cheetos"
'89 K100RS Special Edition "Special Ed"

Offline natalena

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Re: 1985 BMW K100 random miss
« Reply #76 on: January 06, 2019, 09:48:37 AM »
As the pundits vehemently push their technology driven conjecture, there are a select few that stand proud and firmly point to the results that MAF sensor jiggling is directly responsible for, and will correct erratic idle. Is this connected to petroglyphs, Beanie Babies, and dolphin massacres seen the world over?

Ancient alien theorists say ... "YES"
  • MST
  • 1987 K75s #0919, '05 Sportster 1200C
We don't need no stinkin' moly.

Offline warmas

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Re: 1985 BMW K100 random miss
« Reply #77 on: January 06, 2019, 03:03:47 PM »
I had previously tightened all accessible intake system connections, as I had mentioned. I think this tightening in very large measure sealed intake leaks and the smoothed rough idle, rather than, tactfully, airflow meter adjustments which can both cause and smooth erratic idle, in my experience.
Rough idle occured with cold engine, as I mentioned.
Warm engine exhibited no noticeable change below 4k rpm in engine operation, as I mentioned.
Why and how, specifically, does the engine stumble upon oil-filler cap removal?

I wonder how one would measure/improve the thermal efficiency of the engine: Egt, cht, mpg, compression test, spark plugs, timing, top overhaul? I have not explored hotter spark, probably unnecessary. Best power is typically a rich mixture, best economy considerably leaner.

I believe, despite your conclusion to the contrary, I have leaned the entire fuel map rather than enriched it. Mpg figures and wiper base points and spring settings and idle bypass settings  and color of exhaust pipe and plugs support this belief. The base wiper setting is now leaner than stock, and barely moves and causes no rpm change at idle, by observation. As mentioned,  by observation at current tune at speed minute wiper deflection leaner breaks up engine, richer speeds up engine.  Pressing the starter button below approx.3.5k rpm results in rpm increase. I have made these observations with the airflow meter uncovered,  connected to plenum, harness connected, and engine running on center stand. Please post your more precise test apparatus. I do not have a dynamometer. I had previously restored the airflow meter to its original settings, as you recommend, burned a few tankfuls and observed no increase if not a decrease in mpg. A lean fuel map may not increase mpg.
Thus my experiment.
Further reading: https://itinerant-air-cooled.com//viewtopic.php?f=50&t=7761

Oil cap removal at idle resulted in a vapor leak out of, rather than into, the crankcase, which indicates crankcase pressure higher than ambient, and such removal implies, contrary to my assumptions, less rather than more vapor inhaled into plenum via crankcase vent post-oil filler cap removal. I suspect engine idle stumble post-cap removal is due to wiper position change (intake air change  negligeable?). Next time I have bike running and airflow meter on bench consequences of oil-filler cap  and/or vent-to-plenum removal to wiper will be obvious.

The purpose of this exercise is to achieve a smooth running bike and the best possible mpg. Bike does have a full fairing, tall windshield. Neither brakes, bearings, nor transmission drag. Cars pass me regularly. The fun/dollar quotient of this machine is already high, your high mpg figures lure me.


  • Enid, OK
  • 1985 K100 resuscitated, 1980 BMW R100, 1985 BMW R80

Offline daveson

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Re: 1985 BMW K100 random miss
« Reply #78 on: January 06, 2019, 05:29:36 PM »
When you take oil cap off unmetered air enters the engine so the computer doesn't know how much fuel to supply. I'm thinking air enters and exits the cap opening  as the piston's move, a piece of paper over the opening might show that,  but it doesn't matter because the result of that test was good.
  • Victoria, Australia
  • Current;'85 K100RT~100,000km; four other bricks. Past; 1500 Vulcan, V Star 650, KLX 250(dirt bike) TT250(dirt bike)

Offline daveson

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Re: 1985 BMW K100 random miss
« Reply #79 on: January 06, 2019, 05:41:14 PM »
By the way what does it mean if it does or doesn't miss on the centrestand or sidestand.
  • Victoria, Australia
  • Current;'85 K100RT~100,000km; four other bricks. Past; 1500 Vulcan, V Star 650, KLX 250(dirt bike) TT250(dirt bike)

Offline Laitch

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Re: 1985 BMW K100 random miss
« Reply #80 on: January 06, 2019, 05:51:06 PM »
By the way what does it mean if it does or doesn't miss on the centrestand or sidestand.
The center stand or side stand positions shouldn't affect the performance of your motos' engines. If one is parked on the side stand the engine might have a tendency to temporarily smoke on startup.
  • Along the Ridley in Vermont.
  • 1995 K75 89,000 miles
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Offline BearTrap

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Re: 1985 BMW K100 random miss
« Reply #81 on: February 07, 2019, 12:18:35 PM »
I think with your year model the 7l warning was not enabled because many people found it more annoying than useful.

I'm also interested if the revs change at idle if the filler plug is removed, but I'm thinking if the revs doesn't change then the z hose is possibly leaking.

Is there still a miss, sometime this is an indication that the fuel pressure regulator is leaking fuel. Since the miss disappears at high revs this also indicates the regulator as a cause, i think.

I've had the vary same problem with my K75. I replaced the fuel pressure regulator with an after market for a VW (with the same pressure rating), along with replacing the vacuum hose for the regulator. Now purrs without a single miss. Run your engine for a minute or two, turn it off and unplug the vacuum line for the regulator #4 throttle and check for leaking fuel (or smell it for fuel). If present, replace the fuel pressure regulator and vacuum line. good luck
  • Rancho Mirage, CA
  • 1990 K75 low seat

Offline daveson

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Re: 1985 BMW K100 random miss
« Reply #82 on: February 07, 2019, 02:47:33 PM »
Hi bear trap

That's interesting

Which year and model VW has a pressure regulator suitable for our bikes, and which brand did you use?
  • Victoria, Australia
  • Current;'85 K100RT~100,000km; four other bricks. Past; 1500 Vulcan, V Star 650, KLX 250(dirt bike) TT250(dirt bike)

Offline BearTrap

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Re: 1985 BMW K100 random miss
« Reply #83 on: February 07, 2019, 06:15:42 PM »
I used a standard PR134. They are used in a whole slew of vehicles including VW and Porsche. The one I used was for VW/Porsche but it's a generic brand (no name), however 50 bucks brand new it's been running over a year now.

Bosch Part # 0 280 160 200
BMW Part # 13 53 1 460 451
VW/Porsche Part # 022 906 035
Jaguar Part # 73177A
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  • Rancho Mirage, CA
  • 1990 K75 low seat

Offline daveson

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Re: 1985 BMW K100 random miss
« Reply #84 on: February 07, 2019, 08:11:20 PM »
Bonus,  thanks.
  • Victoria, Australia
  • Current;'85 K100RT~100,000km; four other bricks. Past; 1500 Vulcan, V Star 650, KLX 250(dirt bike) TT250(dirt bike)

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