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  1. #1

    1985 K100RS Problems

    Good morning everyone.
    1. It seems that I need a long time to warm up my bike in the morning, have to keep the choke on for about five minutes. Then the bike's RPM reaches from 1500 to almost 3000 in idle at all of sudden. I have to turn off the choke, because I think the RPM is too high for its idle. But the engine will be off as soon as I turn off the choke. So I have to hold my throttle a little bit and keep the RPM under 2000 for another a couple of minutes, otherwise it will die at traffic lights if I ride it.
    2. After coming back from the rally last year, I replaced fuel filter, a damper and two fuel hoses from tank to the rail. But the bike uses much more gas than before. I remembered that I could ride almost two hundred mile with a full tank. But now, less than 150 miles I have to fill it up.
    I replaced spark plugs, ignition wires, thermostat about 30.000 miles ago. Replacing air filter every 20.000 miles and changing motor oil every 3000 miles. I always use BMW original parts and I really don't know what causes the problems.
    The bike is only 47.000 miles on it. Is it suppose to be like that because of the bike's age?
    Anyone has some ideas what's going on here?
    Thanks in advance.

    Sinclair Niu
    Enjoy Riding and Ride Safe
    Sinclair & Mona Hamilton, NJ
    84 Honda Magna 500 / 85 BMW K100RS / K1200LT Will Be Next

  2. #2
    Registered User Beemer01's Avatar
    Join Date
    Mar 2007
    Chicago, Ill

    This sounds like a fuel or hose leak problem

    On these old K bikes - and I owned one for years - air for combustion is seldom the issue. It doesn't sound like a spark problem......

    You seem to have addressed the fuel angle and unless the injectors got clogged - let's look for a leaking hose. There is a breather hose - leaks here cause significant problems and after nearly 30 years yours would seem about due to expire.

  3. #3
    Registered User
    Join Date
    Nov 2009

    Low mileage on K100RS

    Mine had low mileage around 100,000 mi.

    Fuel pressure was way too high, Dealer found and fixed this. Its a fuel pressure regulator, not easy to measure or put in.
    Then I found two of 4 tight exhaust valves, dealer used my shim measurements and his selection of spacer to set valves correctly.

    Between the two, mileage was back.

  4. #4
    Thank you so much for the inputs. There is no fuel leak for sure. But I will defiantly check my fuel pressure first. That's a very good point to start with. Let me find what's the pressure suppose to be and start from there.
    Thank you again.
    Enjoy Riding and Ride Safe
    Sinclair & Mona Hamilton, NJ
    84 Honda Magna 500 / 85 BMW K100RS / K1200LT Will Be Next

  5. #5
    3 Red Bricks
    Join Date
    Jan 2007
    Pleasanton, Ca.
    Fuel pressure should be in the 37-40 range. With a failed regulator, it can get as high as 80-100. The easiest place to check it is to TEE in to the line that goes from the fuel rail to the regulator at the back end of the fuel rail. Make sure you TEE into it, as it still needs to be connected to the regulator.

    Another thing that fits your symptoms it a cracked crankcase vent hose (Beemer01 suggested this). It is a Zee shaped hose about 5/8" diameter and 6" long. It is located just to the rear of the cylinder head and to the inside of the throttle position switch on the rear end of the throttle bodies. The cracks will be right next to the top and bottom hose clamps. This hose vents the crankcase from the top of the block into the intake plenum. When this hose cracks (it always does), it allows unmetered air into the intake (large vacuum leak) and gives the symptoms you described. The hose is cheap $18 and can be changed easily. This would be my first check.


    Ride Safe, Ride Far, Ride Often

    Lee Fulton Forum Moderator
    3 Marakesh Red K75Ss
    Mine, Hers, Spare

  6. #6
    Internet diagnostics are risky, but remember that the "choke" is nothing more than a fast idle switch. I think you have a faulty engine temperature sensor causing a rich condition.
    Paul Glaves - "Big Bend", Texas U.S.A
    "The greatest challenge to any thinker is stating the problem in a way that will allow a solution." - Bertrand Russell

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