BMW K bikes (Bricks)


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timz10000

timz10000
Silver member
Silver member
I formerly posted at this link but thought I'd start a new thread for clarity

My 85 K100 starts running rough after about 10 minutes. I'm wondering what to try next - any suggestions welcome! 

Symptoms: It starts and runs pretty well when cold. After say 10 mins it doesn't cut out completely, but starts to sputter and run really rough. The battery light comes on at times when it's about to die. I can still rev it high and it seems to run better at high rev (but still not great)

When I cut the motor and leave it for not too long (guessing 10 minutes) it'll start up fine again.

I have a parts bike so I can try swapping things (parts bike hasn't run in many years, but was out of the weather - tank not functional tho)

What it's not:
swapped coils
swapped ECU
swapped ignition control unit
fuel lines replaced inside and outside tank
fuel system has pressure / fuel pump working
thermostat in radiator replaced (failed hot water test so I got a working one)
thermostat temp. sensor replaced

What it might be:
I replaced the Hall Sensors themselves but didn't replace the wiring
When tank is low on gas and I open it, vapor comes out - possible vapor lock? (When I open cap I don't get whooshing of air)

Other guesses:
could fuel pressure regulator or vacuum hoses cause this?


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1985 K100
    

MartinW

MartinW
Life time member
Life time member
Sounds like you are getting a vacuum lock in the tank. Try running until it starts to play up and with a spare key pop the fuel cap. If it starts to behave it's self you have found the problem. The cap can be pulled apart and cleaned and the cap service kit is still available.
Regards Martin.

    

timz10000

timz10000
Silver member
Silver member
MartinW wrote:Sounds like you are getting a vacuum lock in the tank. Try running until it starts to play up and with a spare key pop the fuel cap. If it starts to behave it's self you have found the problem. The cap can be pulled apart and cleaned and the cap service kit is still available.
Regards Martin.
Cool thanks will try that tomorrow.


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1985 K100
    

timz10000

timz10000
Silver member
Silver member
timz10000 wrote:
MartinW wrote:Sounds like you are getting a vacuum lock in the tank. Try running until it starts to play up and with a spare key pop the fuel cap. If it starts to behave it's self you have found the problem. The cap can be pulled apart and cleaned and the cap service kit is still available.
Regards Martin.
Cool thanks will try that tomorrow.

I tried it but that wasn't the problem. (Ran for 20 mins til it died, wouldn't start. Waited 15 mins, restarted with the cap open, ran for a few mins then cut out again and wouldn't start).

Any recommendations about what to try next?


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1985 K100
    

MartinW

MartinW
Life time member
Life time member
When it cuts out pull the spark plugs and report whether they are wet or dry.
Regards Martin.

    

92KK 84WW Olaf

avatar
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Life time member
When you say replaced the Hall Sensors, was that new ones or replacement used ones?.......

Funny I started thinking of Fuel pressure regulator too and see you mentioned it.

Battery light glowing on at very low revs is not unusual.

Wonder could that fuel tank connector plug be acting up and causing loss of fuel pressure...? Wiring to that goes alongside and near the radiator pipes on the bolt down early 85 tanks.


__________________________________________________
1992 K100LT 0193214 Bertha Blue 101,000 miles
1984 K100RT 0022575 Brutus Baja Red 578 bought 36,000 now 89,150 miles
1997 K1100LT 0188024 Wotan Mystic Red 689 58,645 now 106,950 miles Deceased.
1983 K100RS 0011157 Fricka 606 Alaska Blue 29,495 miles Damn K Pox Its a Bat outta Hell Now 58,200 miles. 
1996 K1100LT 0233004 Lohengrin Mystic Red 38,000 miles currently 51,800 miles.
1983 K100RS 0004449 Odette R100 colours 58,000 miles. Sprint fairing now 63,390 miles

Past:
1968 Yamaha 80 YG1
1971 Yamaha 125 YAS-1
1968 Honda 125 SS
1970 Honda CD 175
1973 Honda CB500-4
Honda CX 500
    

timz10000

timz10000
Silver member
Silver member
92KK 84WW Olaf wrote:When you say replaced the Hall Sensors, was that new ones or replacement used ones?.......

Funny I started thinking of Fuel pressure regulator too and see you mentioned it.

Battery light glowing on at very low revs is not unusual.

Wonder could that fuel tank connector plug be acting up and causing loss of fuel pressure...? Wiring to that goes alongside and near the radiator pipes on the bolt down early 85 tanks.

With the Hall Sensors I bought new actual sensors, and put them on the plate and wired them to the existing wires (but it seems possible the old wires get hot and maybe fail? I'm thinking I'll try putting in the Hall sensors from my parts bike since the behavior sounds like it could be that.)

I'll try swapping the Fuel pressure regulator. 

With the fuel tank connector plug - the mechanic I took it to said the fuel pressure was good when the bike was cutting out (He thought the problem was probably the ECU or the ignition control unit, but then I tried those and neither fixed it).


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1985 K100
    

Rick G

Rick G
admin
admin
I would be taking a new look at the hall sensors because what you describe is very typical of them.


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"Man sacrifices his health in order to make money.
Then he sacrifices money to recuperate his health.
And then he is so anxious about the future that he does not enjoy the present; the result being that he does not live in the present or the future; he lives as if he is never going to die, and then dies having never really lived."   Dalai Lama


Bikes 1999 K1100 LT with a Big Block 1200
    

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