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Thread: '92 K75RT engine lifespan ?

  1. #1
    Registered User scoobs's Avatar
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    Question '92 K75RT engine lifespan ?

    My k bike has 134,000 miles on it and the gas mileage has really dropped in recent years to a miserable 35 mpg/180 miles per tank. I used to get 50 + mpg/240 miles per tank when I first bought the bike with 70,000 miles on it. A recent inspection showed oil fouling on one of the spark plugs some I'm thinking it might need new piston rings ? If you add ongoing starter sprag clutch problems ( bike runs 2,500 miles before I need to do a Motormedic flush- I feel it needs a proper tear-down to clean things properly. I've been advised that will run around $600 for labour plus any parts that might be needed.) then I'm wondering if it would be more advisable to install a used engine ? I've found one for $450 + $250 shipping that has 55K miles on it- I'd still have to install it but it should solve both problems. It seems a better option from a financial point of view - anyone have any thoughts on this ? I understand the effects of wear and tear but have heard of K's with several hundreds of thousands of miles on them and wondered what was considered normal ?
    Cheers,
    Scoobs.
    Ian Robert "Scoobs" Scobie

    '92 K75RT, '02 F650GS Dakar;
    But fondly miss.. R80RT, R45,CB250RS,DT125MX,TS100

  2. #2
    Benchwrenching PGlaves's Avatar
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    Before I did anything else I would have compression and leakdown tests performed to better determine the condition of the engine. Having put 370,000 on a K75 before it was wrecked I consider engine problems at 134,000 as premature. If that one cylinder comes up with what seems to be a ring leak then I would try to get that cylinder examined with a borescope to see if damage to the Nikasil cylinder was evident. This is rare but has happened. If that can be determined then a different engine is the way to go. Otherwise, it depends on what the results of theleakdown test are.
    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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  3. #3
    Mars needs women! 35634's Avatar
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    +1 on a leakdown test. Anything is possible, just because some engines go forever does not mean others won't go belly up a 100k. Maybe the original owner used the wrong brand/weight/type oil Generally, I would say old bricks succumb to other issues before the engine wears out. Replacing with a used is is a bit of a gamble but probably much more cost effective than rebuilding a bad one.
    1987 K75S
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  4. #4
    RK Ryder
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    My '87 K100 has 154,000 miles and gets only slightly less gas mileage as when I bought it at 77,000 miles. There was a time when the mileage dropped, but then I realized that my right wrist was becoming much less inhibited than during my first three years of riding it. Once I got back to smoothly rolling on the throttle, the gas mileage returned close to what it used to be.
    Paul
    Retired and riding my RTs, the '87 K100 & the '98 R1100 !
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  5. #5
    Dances With Sheep GREGFEELER's Avatar
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    Quote Originally Posted by scoobs View Post
    My k bike has 134,000 miles on it and the gas mileage has really dropped in recent years to a miserable 35 mpg/180 miles per tank. I used to get 50 + mpg/240 miles per tank when I first bought the bike with 70,000 miles on it. A recent inspection showed oil fouling on one of the spark plugs some I'm thinking it might need new piston rings ? If you add ongoing starter sprag clutch problems ( bike runs 2,500 miles before I need to do a Motormedic flush- I feel it needs a proper tear-down to clean things properly. I've been advised that will run around $600 for labour plus any parts that might be needed.) then I'm wondering if it would be more advisable to install a used engine ? I've found one for $450 + $250 shipping that has 55K miles on it- I'd still have to install it but it should solve both problems. It seems a better option from a financial point of view - anyone have any thoughts on this ? I understand the effects of wear and tear but have heard of K's with several hundreds of thousands of miles on them and wondered what was considered normal ?
    Cheers,
    Scoobs.
    Don't discount the effect of Ethanol fuel on your mileage. In the area where I live, I get a solid 5 MPG less in all my bikes than when I buy fuel in several areas I travel through which are about 400 miles from here. This has been consistent for the last five years or so, but was not always the case. Also, have you have had the CO adjustment done, or checked the air bypass hose, or any of the other rubber bits in the intake track?

    The sprag clutch is a puzzle. What motoroil do you use?
    Greg Feeler
    BMW MOA Director & Ambassador
    1972 R75/5, 1990 K75, 1990 K1, 1992 K75S, 2003 K1200RS

  6. #6
    Registered User scoobs's Avatar
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    Thumbs up

    After chatting with a service tech. today I'm going to replace the water temperature sensor- I know it works as the cooling fan cut on several times this afternoon while idling in traffic, however was told that if it is adrift by even 10 degrees it can cause the mixture to run too rich and increase fuel consumption. I did test it last year by measuring the changing output resistance as the engine temperature increased but didn't accurately track the actual values- just kind of "ballparked the figures" to prove it varied with temperature change.
    I'm aware that ethanol in the gas can reduce mpg by around 5% but I'm seeing a difference of almost 15 mpg so believe there is something wrong somewhere. I don't consider myself a speed merchant and normally ride fairly smoothly- however am prone to cracking it open for overtaking etc.
    Thanks for your comments, I'll keep you updated once I get the new sensor installed.
    Cheers,
    Scoobs.
    Ian Robert "Scoobs" Scobie

    '92 K75RT, '02 F650GS Dakar;
    But fondly miss.. R80RT, R45,CB250RS,DT125MX,TS100

  7. #7
    A bozo on the bus deilenberger's Avatar
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    Quote Originally Posted by scoobs View Post
    After chatting with a service tech. today I'm going to replace the water temperature sensor- I know it works as the cooling fan cut on several times this afternoon while idling in traffic, however was told that if it is adrift by even 10 degrees it can cause the mixture to run too rich and increase fuel consumption. I did test it last year by measuring the changing output resistance as the engine temperature increased but didn't accurately track the actual values- just kind of "ballparked the figures" to prove it varied with temperature change.
    I'm aware that ethanol in the gas can reduce mpg by around 5% but I'm seeing a difference of almost 15 mpg so believe there is something wrong somewhere. I don't consider myself a speed merchant and normally ride fairly smoothly- however am prone to cracking it open for overtaking etc.
    Thanks for your comments, I'll keep you updated once I get the new sensor installed.
    Cheers,
    Scoobs.
    The water temperature sensor you may need to replace is the one on the F shaped pipe coming off the top of the engine block. It is a dual element sensor - meaning - the fan working doesn't mean the ECU is seeing the correct engine temperature. They work off different elements in the sensor.

    To measure the temperature values for the ECU - it's best done at the ECU plug. Someplace on the IBMWR K-tech pages there is a chart that shows water temperature vs resistance, and tells you which pin on the connector to measure at. Be VERY careful not to damage the fingers on the ECU connector - they are easily bent and about impossible to repair. The official way of probing that connector is to remove the back shell, and go in with something like an unfolded paperclip next to the wire until it touches where the wire is crimped into the finger assembly. Testing at this point means the entire circuit is tested. The measurement is taken from the connector pin to ground. On the sensor itself - the two halves are the output pins, ground is the shell.
    Don Eilenberger http://www.eilenberger.net
    Spring Lk Heights NJ NJ Shore BMW Riders New Sweden BMW Riders
    '07 R1200R (current ride) and some bimmers.. and a Porsche

  8. #8
    A bozo on the bus deilenberger's Avatar
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    Oh - one other thought. When is the last time you checked the valve adjustment? Too tight valves can cause a loss of power and excessive fuel use.
    Don Eilenberger http://www.eilenberger.net
    Spring Lk Heights NJ NJ Shore BMW Riders New Sweden BMW Riders
    '07 R1200R (current ride) and some bimmers.. and a Porsche

  9. #9
    Dances With Sheep GREGFEELER's Avatar
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    Quote Originally Posted by deilenberger View Post
    The water temperature sensor you may need to replace is the one on the F shaped pipe coming off the top of the engine block. It is a dual element sensor - meaning - the fan working doesn't mean the ECU is seeing the correct engine temperature. They work off different elements in the sensor.

    To measure the temperature values for the ECU - it's best done at the ECU plug. Someplace on the IBMWR K-tech pages there is a chart that shows water temperature vs resistance, and tells you which pin on the connector to measure at. Be VERY careful not to damage the fingers on the ECU connector - they are easily bent and about impossible to repair. The official way of probing that connector is to remove the back shell, and go in with something like an unfolded paperclip next to the wire until it touches where the wire is crimped into the finger assembly. Testing at this point means the entire circuit is tested. The measurement is taken from the connector pin to ground. On the sensor itself - the two halves are the output pins, ground is the shell.
    I HIGHLY recommend using that chart for the resistance values of the temp sensor as Don says. It saved me from doing a while lot of work when the problem turned out to be something else. Here's the link: http://www.ibmwr.org/ktech/gas-mileage.shtml
    Greg Feeler
    BMW MOA Director & Ambassador
    1972 R75/5, 1990 K75, 1990 K1, 1992 K75S, 2003 K1200RS

  10. #10
    Dances With Sheep GREGFEELER's Avatar
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    Quote Originally Posted by scoobs View Post
    After chatting with a service tech. today I'm going to replace the water temperature sensor- I know it works as the cooling fan cut on several times this afternoon while idling in traffic, however was told that if it is adrift by even 10 degrees it can cause the mixture to run too rich and increase fuel consumption. I did test it last year by measuring the changing output resistance as the engine temperature increased but didn't accurately track the actual values- just kind of "ballparked the figures" to prove it varied with temperature change.
    I'm aware that ethanol in the gas can reduce mpg by around 5% but I'm seeing a difference of almost 15 mpg so believe there is something wrong somewhere. I don't consider myself a speed merchant and normally ride fairly smoothly- however am prone to cracking it open for overtaking etc.
    Thanks for your comments, I'll keep you updated once I get the new sensor installed.
    Cheers,
    Scoobs.
    I have to say that I've never gotten 50mpg or even close to that with a K75. However, there are different riding styles. How would you describe your riding style, and how long ago was it when you did get those kind of mileages?
    Greg Feeler
    BMW MOA Director & Ambassador
    1972 R75/5, 1990 K75, 1990 K1, 1992 K75S, 2003 K1200RS

  11. #11
    3 Red Bricks
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    Quote Originally Posted by deilenberger View Post
    Oh - one other thought. When is the last time you checked the valve adjustment? Too tight valves can cause a loss of power and excessive fuel use.
    A VERY big +1 !!!!



    LONG MAY YOUR BRICK FLY!

    Ride Safe, Ride Far, Ride Often

    Lee Fulton Forum Moderator
    3 Marakesh Red K75Ss
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  12. #12
    A bozo on the bus deilenberger's Avatar
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    Quote Originally Posted by GREGFEELER View Post
    I have to say that I've never gotten 50mpg or even close to that with a K75. However, there are different riding styles. How would you describe your riding style, and how long ago was it when you did get those kind of mileages?
    That was my experience with my K75S - it actually got a bit worse mileage than my '85 K100RT did, and that was fairly awful. On average - I'd guess around 38MPG unless I was on a trip and stayed near the speed limit, then it would just barely get 40MPG.

    I actually ride considerably faster on my current bike (R1200R) and get much better mileage. It's not hard to get 45MPG from it, and I've seen 55MPG on a trip. The difference is a much smarter fuel injection system. It also makes a bunch more HP..
    Don Eilenberger http://www.eilenberger.net
    Spring Lk Heights NJ NJ Shore BMW Riders New Sweden BMW Riders
    '07 R1200R (current ride) and some bimmers.. and a Porsche

  13. #13
    Dances With Sheep GREGFEELER's Avatar
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    Quote Originally Posted by deilenberger View Post
    That was my experience with my K75S - it actually got a bit worse mileage than my '85 K100RT did, and that was fairly awful. On average - I'd guess around 38MPG unless I was on a trip and stayed near the speed limit, then it would just barely get 40MPG.

    I actually ride considerably faster on my current bike (R1200R) and get much better mileage. It's not hard to get 45MPG from it, and I've seen 55MPG on a trip. The difference is a much smarter fuel injection system. It also makes a bunch more HP..
    I can get 40 to 45 with my K75S at 75 to 80, but pushing above that or bucking a head or side wind and it will drop to the high 30's. But, that gives me about 200 miles to a tank which is acceptable, and I just love that bike.
    Greg Feeler
    BMW MOA Director & Ambassador
    1972 R75/5, 1990 K75, 1990 K1, 1992 K75S, 2003 K1200RS

  14. #14
    Nick Kennedy
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    My 1990 K75RT gas mileage varies quite a bit due to different riding styles and speeds, of course.
    Cruising steadily at say 65-70 it averages about 40- 44 Mpg
    Riding with friends in the canyons with a lot of full throttle bursts and full throttle running it can easily get down to 30- 35 Mpg
    This is with a well tuned engine.

  15. #15
    Mars needs women! 35634's Avatar
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    Quote Originally Posted by GREGFEELER View Post
    I can get 40 to 45 with my K75S at 75 to 80, but pushing above that or bucking a head or side wind and it will drop to the high 30's. But, that gives me about 200 miles to a tank which is acceptable, and I just love that bike.
    Mine too, and I can touch 50mpg if I can hold 65mph, but it has dropped some in it's old age. Nobody has addressed the fouled plug yet. I'd be curious to see a compression test at least.
    1987 K75S
    Original litter
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    2012 Ural Gear Up

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