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Thread: low fuel mileage on 88 K75C

  1. #1
    Registered User rall275's Avatar
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    low fuel mileage on 88 K75C

    I have a 1988 K75C with relatively low miles. Bought it last summer, changed all fluids, new plugs. I've been riding it and my mileage last check was 34.5 mpg.

    Rides well. When warm outside, takes multiple attempts to fire off.

    What do you think's causing the low mileage? Does it need a valve adjustment? Fuel injector synching?

    Thanks guys!

    Keith
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  2. #2
    My guess is that you have a rich condition, probably caused by a faulty engine temperature sensor. You can check the mixture condition by doing a "chop test."

    Find a suitable rural road and ride the bike at highway speeds for several miles. Arrive at a pre-planned location where you can kill the engine at highway speed and coast to a stop at a good, safe location. Rural convenience stores or church lots, or rest areas work well. Let it cool and then remove the spark plugs. You will want to see a light tan insulator with no obvious deposits. If bright white the mixture is lean. If there is any sign of darkness or black fluff it is too rich.
    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
    http://web.bigbend.net/~glaves/

  3. #3
    Registered User mlytle's Avatar
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    gotta do a complete baseline tuneup.

    clean tb's
    valve check/adjust
    tb synch
    mixture adjust
    injector clean and balance
    air filter
    fuel filter
    vacuum leak check
    temp sensor check can be done with an ohmmeter.

    then make sure driveline is in good shape to eliminate extra drag. spline lube, tire pressure, front wheel bearing grease, etc.

    these things are old. assume prior owners did nothing, do a full nose to tail refresh.

    mine gets 49-50mpg.

    see here
    http://www.motobrick.com/index.php/topic,7810.0.html
    Marshall
    92 K75s
    94 K75s
    09 K1300s

  4. #4
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    Keith,

    Start with checking the crankcase vent hose for cracks.
    Then inspect the two vacuum caps on #1 and #2 throttle bodies.
    Then make sure that the nipple on the front top of the block (right below the fan) is either securely capped or is connected with uncracked hose to the forward nipple at the right rear of the fuel tank (with the factory check valve in the middle of the hose.
    Check the O-ring on the oil filler cap. It should touch before the plastic bottoms on the crank cover.

    Then check the valve adjustment. If you haven't checked the valve adjustment since you bought it, it needs to be checked. Write down your dated results for future reference. You don't need any special tools, other that a set of feeler guages and an inch pound torque wrench (or a very light touch) for the valve cover bolts, to CHECK the adjustment, only to change things. Exhaust valves should be set to the loose limit. If they are at .010", set them to .012". Replace all 10 of the rubber bushings on the valve cover bolts (less than $1 each). Use an inch/pound torque wrench to tighten the valve cover bolts to only 78 INCH pounds.


    After any valve adjustment, you should do a throttle body balance. Only adjust the large brass bypass screws. DO NOT TOUCH THE SCREWS BETWEEN THE THROTTLEBODY LINKAGE!!!

    These things need to be done to any early K that you've just purchased. If they don't correct your problem, you'll need to go further, but we will know that these important things are correct and it will be easier to diagnose.





    LONG MAY YOUR BRICK FLY!

    Ride Safe, Ride Far, Ride Often

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

  5. #5
    Registered User rall275's Avatar
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    Thanks guys for the info. I'm not the greatest mechanic, but will continue to research where all these items are and go from there. There's a video on how to check valve clearances, so I can probably manage that. I'll look for those hoses and see if I can replace some of them. I envy you guys who know how to tackle these mechanical issues--would be a blast to be able to master these!

  6. #6
    Registered User rall275's Avatar
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    Hall sensor on 88 K75C

    I doubt it has anything to do with the low mileage, but that my bike has difficulty starting when it's hot outside, suggests that it might be a hall sensor. I've looked at my Haynes and Clymer's, but how difficult a job is this anyhow?

  7. #7
    Rally Rat 1074's Avatar
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    fuel mileage

    I know that it's an off chance but have you adjusted the idle mixture? This procedure uses an exhaust gas analyzer with the "sniffer" stuck up the muffler. It is used to measure and adjust CO emissions. This adjustment pertains to lean or rich conditions. The procedure is outlined in both the Clymer and Haynes manual.
    The only reason I mention this is that this was a check point for assembly and pre-delivery. Some dealers (not mentioning any names) were either too thrifty or to confused by this modern technology, to perform this test. Knowing this, when I bought my first K75 the first thing I did was have this point checked. The adjustment was way off!
    Boxerbruce

  8. #8
    Registered User rall275's Avatar
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    Thanks for the tip, I'll add it to my list of things to check. My first priority is to check the valve clearances and replace fuel/vacuum hoses.

  9. #9
    Registered User K75solo's Avatar
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    Idle

    Quote Originally Posted by 1074 View Post
    I know that it's an off chance but have you adjusted the idle mixture? This procedure uses an exhaust gas analyzer with the "sniffer" stuck up the muffler. It is used to measure and adjust CO emissions. This adjustment pertains to lean or rich conditions. The procedure is outlined in both the Clymer and Haynes manual.
    The only reason I mention this is that this was a check point for assembly and pre-delivery. Some dealers (not mentioning any names) were either too thrifty or to confused by this modern technology, to perform this test. Knowing this, when I bought my first K75 the first thing I did was have this point checked. The adjustment was way off!
    Yeah, I had a hard time getting the warm idle right and didn't have the sniffer you mentioned, so asked a mechanic if the symptoms were those of rich or too lean. He said too rich so I turned it about 1/8 a turn and think it helps. But as you say, the only real solution is to have the exhaust gas analyzed, apparently. The manual says to not adjust this much as it's sensitive. I wish I had a competent mechanic here in northwest Wyoming... have to be sort of self-reliant in these parts. BMW Denver doesn't even want to mess with these old K75 bikes.

  10. #10
    Rally Rat 1074's Avatar
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    competent mechanic

    Quote Originally Posted by pippen View Post
    Yeah, I had a hard time getting the warm idle right and didn't have the sniffer you mentioned, so asked a mechanic if the symptoms were those of rich or too lean. He said too rich so I turned it about 1/8 a turn and think it helps. But as you say, the only real solution is to have the exhaust gas analyzed, apparently. The manual says to not adjust this much as it's sensitive. I wish I had a competent mechanic here in northwest Wyoming... have to be sort of self-reliant in these parts. BMW Denver doesn't even want to mess with these old K75 bikes.
    Sure, a good mechanic is hard to find but as outlined, the adjustment is pretty easy. All you need is someone with an exhaust gas analyzer, a little patience and a 5mm allen wrench. And yes, it is a touchy adjustment.
    Boxerbruce

  11. #11
    Registered User drneo66's Avatar
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    Quote Originally Posted by rall275 View Post
    Thanks guys for the info. I'm not the greatest mechanic, but will continue to research where all these items are and go from there. There's a video on how to check valve clearances, so I can probably manage that. I'll look for those hoses and see if I can replace some of them. I envy you guys who know how to tackle these mechanical issues--would be a blast to be able to master these!
    You got this! We happened to be blessed with a lot of knowledgeable people here on this forum, so keep posting if you get in a bind.

    I assume you have a good manual? BMW, Clymer, Etc?

    Have you looked in the Anonymous book for people willing to help?

    Turning one bolt is not different than turning any other - so just start with one
    1995 BMW K75s - 100k and climbing!
    2007 BMW R1200RT - 62k
    2009 BMW G650GS - 22k and ready for Alaska!

  12. #12
    Registered User K75solo's Avatar
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    I will answer

    Quote Originally Posted by drneo66 View Post
    You got this! We happened to be blessed with a lot of knowledgeable people here on this forum, so keep posting if you get in a bind.

    I assume you have a good manual? BMW, Clymer, Etc?

    Have you looked in the Anonymous book for people willing to help?

    Turning one bolt is not different than turning any other - so just start with one
    His twin brother here... I can answer for him, you know. Anyway, he has a Clymer's and I gave him some names out of a 2015 Anonymous book I have, so you are on the same wave length. I told him to just start with the easy things first and do research and follow the advice of the experts on this site. One step - one bolt - at a time. Hopefully he won't be spending so much of his time off working on it and gets it in good running form quickly.

  13. #13
    Thick As A Brick r184's Avatar
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    Quote Originally Posted by rall275 View Post
    I have a 1988 K75C with relatively low miles. Bought it last summer, changed all fluids, new plugs. I've been riding it and my mileage last check was 34.5 mpg.

    Rides well. When warm outside, takes multiple attempts to fire off.

    What do you think's causing the low mileage? Does it need a valve adjustment? Fuel injector synching?

    Thanks guys!

    Keith
    Don't forget to check your right wrist too. I've found when my mileage starts to drop it is directly tied to the twisting of the throttle I do with my right wrist. The more and faster I twist that throttle, the more my mileage drops. Real easy thing to do on a K bike.
    (and before the replies start to fly in, yes I am joking)

  14. #14
    Registered User rall275's Avatar
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    I ordered that valve adjustment tool from polepenhollow@yahoo.com, and they look great. He also sent me some detailed photos and an instruction sheet. If the other businesses I deal with were as squared away and empathic as your typical small business beemer guy, I'd spend all of my paychecks on bikes and gizmos!

    Got some wurth K75 feeler gauges (.15, .2, .25, 3) from ebay. Ordered some valve cover gaskets and grommets from Bob's. Max's won't take my CC number.

    Ordered some fuel and vacuum hoses from eEuroparts to replace hoses that might be leaking and are definitely old. 3.2mm ID x 2mm wall silicone vacuum hose (SKU: 11747797175 - Qty: 1 meter - Price (ea): $2.99) and 8x13 fuel hose (SKU: 161211804095EC - Qty: 1 meter - Price (ea): $10.49). These hoses cross referenced to the higher priced bmw hoses.

    So, I'm doing my part to get my bike running just right. Any leads on how to get and replace a hall sensor would also be appreciated. Thanks for your help, and thanks to my twin brother for answering for me.

    After I get all the above fixed (I can dream!), I'll send in my injectors for cleaning after getting all your advice! Haha!

    Keith
    Last edited by rall275; 04-29-2017 at 02:15 PM. Reason: hose size wrong in original

  15. #15
    3 Red Bricks
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    Keith,


    Just a suggestion, since you are new to working on these bikes, do one job/ system/ area at a time and then test ride the bike. If something doesn't work out, you will know the problem lies with what you worked on last. If your original symptoms are cured, you will know that what ever you fixed last was the culprit.

    Remember, You want the exhaust valves at the loose side of the spec. .30mm or .012" The shims come in .05mm or .002" increments. So if the valve is at .25mm/.010" or tighter, change it. You can leave it if it is at .275/.011". Intakes are not as critical. Anywhere within spec is OK.

    You said you replaced the spark plugs. If you used the Bosch X5DC plugs, no problem, but if you used ANY plug with an "R" in the number, they are incorrect and could cause problems. Also some NGK plugs come without the correct tip on the wire end of the plug ( just a threaded stud sticking out) and will definitely not work unless the correct tips are added. In both these previous cases, extra resistance is being added to the plug voltage and the result is poor performance/milage and possible stalling.

    If you are sure that the plugs are correct and good, don't bother to remove them when checking the valves. A K75 will almost always stop its rotation (because of compression and spring pressure) with one intake and one exhaust in the correct position for checking (cam lobe pointin directly away from the valve). Make a chart for your six valves. Remove valve cover and check and write down the clearances of the two valves that the cam lobes are pointing directly away from the valves. Then turn on the key and kill switch, bump the starter with one hand and hit the kill switch with the other. If you are quick, the cams will rotate to where two different valves are ready to be checked. Check and write down the clearances. Do it once more and you are done. If you are not quick with the kill switch, the bike will start and oil will get everywhere. That's why you use TWO hands. Insert the feeler gauges from the spark plug side of each cam.

    After you have the clearances of all the valves writen down, if any valve needs correcting, compressed air works best to unseat the old valve shim from the bucket. You can rotate the bucket by hand when the cam or valve tool is not touching it. There are two small notches in the bucket lip. Direct the air stream there after you've compressed the bucket using the valve tool. A small magnetic retreiving tool will help you fish out the shim. A $10 (on sale) Harbor Freight 6" digital caliper will allow you to measure the shim thickness and convert back and forth from metric to inches. Then you can calculate what shim you need to install. If more than one valve needs correction, sometimes you can use the old shim from one valve to correct another valve thereby saving having to buy too many shims.

    I would hold off on the hall sensor until all the basic stuff is correct. The easiest way to test the hall sensor is to remove the cover and direct a hair drier (not a heat gun. You'll melt the wire insulation if your not careful) right at it. If its bad, the bike will start to miss or die.






    LONG MAY YOUR BRICK FLY!

    Ride Safe, Ride Far, Ride Often

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

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