BMW K bikes (Bricks)

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1Back to top Go down   Another K100 running lean Empty Another K100 running lean on Sat Aug 29, 2020 9:51 pm

Michael Sydney

Michael Sydney
Silver member
Silver member
Yes, following my rich running problem back in July (it turned out to be a blocked return pipe fitting on the tank causing excessive fuel pressure) I now have a fuel starvation problem!

1983 K100, no modifications and all seemingly fine. After fueling up and 100km, including 110kph hwy cruising, it partially lost power. I stopped the motor and could see no problems, restarted and all good..... for another 20 km.
Again the bike slowed, felt like less than 1/4 throttle but cable fine. In neutral I blipped the throttle a few times and normal power returned. This continued for another 50km but throttle blipping no longer effective. Pottered home on what felt like 50cc.
Each time no rough running, no missing, still would Idle smoothly. I removed spark plugs, all show lean mixture, all 4 the same. Fuel pressure a steady 32psi.

If this had a carb I would diagnose blocked main jet! Fitting a new fuel filter has made no difference. All hoses are good and free of kinks. The tank vent works fine so not a tank vacuum. 

Obviously the fuel mapping would not allow such a drop in performance regardless of temperature sensor. Can I only suspect the injection control unit as faulty? Suggestions please Smile


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Living in the past! K100 '83 and R65 '83.
    

2Back to top Go down   Another K100 running lean Empty Lean on Sat Aug 29, 2020 11:20 pm

daveyson

daveyson
Life time member
Life time member
Air flow meter?


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11/1985 bmw k100rt (late model)  Vin. 0090567
    

3Back to top Go down   Another K100 running lean Empty Re: Another K100 running lean on Sat Aug 29, 2020 11:42 pm

Point-Seven-five

Point-Seven-five
Life time member
Life time member
@daveyson wrote:Air flow meter?
+1

Air leak down stream of the air flow meter?

Is something causing fuel starvation.

Fuel pressure regulator?

Fuel pump?

Fuel filter?


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Present:
1994 K75RT
1991 K100RS
1988 K100RS SE

Past:
1994 BMW K75S
1992 BMW K100RS
1982 Honda FT500
1979 Honda XR185
1977 Honda XL125
1974 Honda XL125
1972 OSSA Pioneer 250
1968 Kawasaki 175
    

4Back to top Go down   Another K100 running lean Empty Re: Another K100 running lean on Sun Aug 30, 2020 12:06 am

Laitch

Laitch
Life time member
Life time member
The simplest measure would be to disconnect the Jetronic plug, clean its pins and sockets with DeoxIT, reconnect it and verify that it cannot be loosened.

The same might be done for the ignition control unit.

Have you tested the units with a multimeter using the troubleshooting guide data?


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1995 K75 77,000 miles
Another K100 running lean Usa-lo10
    

5Back to top Go down   Another K100 running lean Empty Re: Another K100 running lean on Sun Aug 30, 2020 12:55 am

Michael Sydney

Michael Sydney
Silver member
Silver member
All good suggestions thank you. I will try working through the diagnosis methodically. 

The only clues I have now is that the throttle blips seem to have had an effect (maybe "exercising" air mass meter?) and a rest seems to restore to normal (so possible heat related).

After changing fuel filter it appeared fixed on my 50 km test ride. Next day I was 80 km from home and it slowed to 110kph max and again I rode slowly home.


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Living in the past! K100 '83 and R65 '83.
    

6Back to top Go down   Another K100 running lean Empty Re: Another K100 running lean on Sun Aug 30, 2020 1:09 am

MartinW

MartinW
Life time member
Life time member
The return line from the FPR can collapse where it enters the tank especially if non OEM on hot days or in traffic. Also check all the other fuel lines for connections and kinks.
Regards Martin.


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K75s Hybrid
    

7Back to top Go down   Another K100 running lean Empty Re: Another K100 running lean on Sun Aug 30, 2020 4:39 am

charlie99

charlie99
VIP
VIP
could the clip around the air filter box piping to plenum have been loose and sucked in one edge of that hose ?

or partially blocked injectors - fuel rail , after that return line failure 

if it could be injectors , possible to remove them and clean the filters on the fuel rail input side ? with out too much drama .

just a couple of suggestions


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cheezy grin whilst riding, kinda bloke ....oh the joy !!!! ...... ( brick aviator )

'86 K100 RT..#0090401 ..."Gerty" ( Gertrude Von Clickandshift ) --------O%OAnother K100 running lean Au-log10

'86 k100 rs.. #######..  "Fred " (f(rame) red ) ( Fredrick leichtundschnell ) - -
bits and pieces from many kind friends across the k100 world ...with many thanks ..
    

8Back to top Go down   Another K100 running lean Empty Re: Another K100 running lean on Sun Aug 30, 2020 9:30 am

Michael Sydney

Michael Sydney
Silver member
Silver member
Again, thank you. 

Surely a return line blockage causes a rise in fuel rail pressure? (as I experienced with a blocked fuel return spigot at fuel tank). Fuel pressure appears to be normal at approx 32 psi

Of significance is the smooth idle even when in fault mode. All four pots equally lean.

I will have a look at the wiper in the air mass sensor.... Maybe it only records a fraction of the actual air flow and fuels to suit. I should be able to test resistance over the whole range of the flap movement. Or as suggested, high resistance on the air mass reading reaching the control unit.


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Living in the past! K100 '83 and R65 '83.
    

9Back to top Go down   Another K100 running lean Empty Re: Another K100 running lean on Sun Aug 30, 2020 10:16 am

moriarti

moriarti
Life time member
Life time member
Thinking out of the box,the ignition control unit under the tank might be the problem old age+heat can cause strange gremlins,worth a look/test to be sure Smile


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1984 k100 rs red/black VIN  0004449
    

10Back to top Go down   Another K100 running lean Empty Re: Another K100 running lean on Mon Aug 31, 2020 3:37 am

Michael Sydney

Michael Sydney
Silver member
Silver member
Well Laitch, I did as you suggested. The earth from Air Mass Sensor and Fuel control unit was a little high resistance. A little over 0.5 ohm, dropping to 0.2 after a thorough clean (tested at multi plug). 

Charlie has an uncanny insight with the suggestion that the hose clamp between air box and plenum might be loose! However, the hose was a tight fit but maybe not so when hot?

I have cleaned all significant connectors as again they can change considerably with heat. I need to test ride but first failure was 100km from home, second about 60 km, so an extended test is called for (though I am not permitted to leave the state Smile )


__________________________________________________
Living in the past! K100 '83 and R65 '83.
    

Michael Sydney

Michael Sydney
Silver member
Silver member
A 400km round trip today, including a couple of hours at motorway speeds, presented no problems. Smile

Hopefully all my connector cleaning had chased the gremlins away. Time will tell.


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Living in the past! K100 '83 and R65 '83.
    

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