K12RS won't rev above 2k.

Discussion in 'Road Warriors' started byQuinn Otte,Oct 23, 2021.

  1. Quinn Otte

    Quinn OtteWrenching > Riding

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    Aug 31, 2015
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    Bike had been sitting a long time and has not run since being assembled ground-up. I've only done the finishing touches, but can't get it tuned right.
    The bike idles well, but if i try to bring it above 2k RPM, it will cut out and miss a few revs, before catching again and surging back to 2k, where it will cut again, oscillating like that at constant throttle until I either open the throttle further and it flames out, or close the throttle so it wants to go back to an idle or sustainable sub-2k engine speed.

    Any ideas?

    Viddo:
    #1
  2. fastring

    fastringLong timer

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    Check the basics:
    - old fuel? If sitting, drain tank and start fresh
    - throttle cable, is throttle moving 100% open? That has a weird 1 to 4 medusa cable thing, worked fine on my 12rs but always looked like a potential weak area
    - plugs? Ez to change so might as well.
    - tight and clean bty connections
    #2
  3. Quinn Otte

    Quinn OtteWrenching > Riding

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    Fuel is fresh, as is the fuel pump.
    The throttle cable actuates a lever on the throttle bodies which opens them all. It is working well.
    Plugs I have not changed, but all 4 cylinders are firing at idle.
    I will check for leaks at the TBs, that's a good call. I bet I can do the aresol spray test at idle.
    #3
  4. Tim McKittrick

    Tim McKittrickLong timer

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    Fuel pressure regulator? At idle and low rpm it will be mostly open to keep the pressure at the injectors constant at low demand- if it is stuck in this position as rpm and load increases the fuel pressure will drop and there won’t be enough fuel to the injectors to let the revs continue to climb.
    #4
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  5. Quinn Otte

    Quinn OtteWrenching > Riding

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    That was a good idea, and I actually had an extra regulator on hand, so I swapped them and put a fuel pressure in-line to monitor. Fuel pressure held right around 55 psi, manual specs 50.6.
    I changed plugs too, no improvement.
    I tried the brake cleaner spray test, and it didn't seem to have an effect.

    Maybe the brain switches maps at 2k and the higher rev one is corrupted? Or a bad TPS?
    #5
  6. Moronic

    MoronicLong timer

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    Shot in the dark but do these use a mass flow sensor in the intake tract like the early bricks did (IIRC)? For informing the fuel map?

    如果是,是否有可能有一个默认的闲置的燃料setting that works, but when you open the throttles a dud or poorlyconnected sensor doesn't add any fuel, and so it leans out and dies?
    #6
  7. Quinn Otte

    Quinn OtteWrenching > Riding

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    I'm leaning toward this being the case, OR a similar issue in the ECU. I need to investigate what sensor feeds what rev range/throttle position. I have a lot of experience on the old motronic K100, but the sensors on the K1200 are way different.
    #7
  8. H96669

    H96669A proud pragmatist.

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    Brakee cleaner isn't too recommended to check vac leaks on the K1200RS, usually says on the can not to use on rubber as it will dry it out and may crack later.

    Same with the plastic bits and the intake manifolds altough the last may just warp then leak.

    I am pretty sure your bike has contaminated fuel injectors.I could test that with the pulser and gauge but easiest for you is to send the injectors out for an ultrasonic cleaning/balance and flow test before & after.Well worth the ~$100.
    #8
  9. Quinn Otte

    Quinn OtteWrenching > Riding

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    I pulled the rail and injectors out and all 4 seemed to spray fine when I hit the starter. But you are right, maybe they're only enough to keep it happy at idle.
    I'll throw them in my US cleaner and see what happens.
    #9
  10. H96669

    H96669A proud pragmatist.

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    They pulse them at high RPMs during the cleaning process.Anything less and they cannot be cleaned successfully.

    Mind you one could buy a cheap pulser on fleabag, mine was ~$30.00 and would pulse 4 injectors.And at the very least will let you do a power balance test with injectors installed and fuel pressure gauge.

    但5%失衡喷油器和engine will run annoyingly buzzy.
    #10
  11. Quinn Otte

    Quinn OtteWrenching > Riding

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    I cleaned the injectors with a pressure/pulse system, then reinstalled. The bike ran GREAT for about 30 seconds, then started the exact same behavior of not revving over 2k. I may need a new fuel filter, flushed system, and clean injectors.
    #11
  12. szurszewski

    szurszewskiLong timer

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    I had a fairly similar problem with a k1200 a couple of years ago after changing the injectors. I could get it go over 2k but it really didn’t want to and was way down on power even at higher RPM. Turned out some idiot (me) had forgotten to to reconnect the throttle position sensor (at/near the end of the fuel rail). Worked much better after plugging that back in.

    There is also a throttle position “re-learn” procedure on these bikes - have you done that since putting it back together? I don’t think it would make this much of a problem, though.
    #12
  13. H96669

    H96669A proud pragmatist.

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    I did something like that, quickly re-contaminated my freshly cleaned injectors from not cleaning the fuel lines.

    Filters are ~15-20 microns.As per Bosch particles of less than 10 microns will contaminate injectors.

    Cut the old filter? If any dirt around the outlet?
    #13
  14. Quinn Otte

    Quinn OtteWrenching > Riding

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    I just pulled the entire fuel system apart and have it cleaning right now. Anything that can fit in my Ultrasonic Cleaner I threw in there, blush and blew out all the lines, flushed the tank with water, then a couple rinses of gas. I have a new filter on the way. I'll clean the injectors again and then give it a test once the new filter gets here
    #14