YZF750R misfire at 4000rpm - help!

Discussion in 'UK Motorcycles' started by Som, Feb 12, 2004.

  1. Som

    Som Guest

    My 1994 YZF750R has developed a misfire at exactly 4000rpm. It won't
    rev beyond 4000 in fact. Absolutely fine up to 4000. So I can zoom
    around at all of about 55mph. This misfire was intermittent for a
    while and would 'clear' (power suddenly comes in and revs to redline
    no problem) after a few seconds. Now it doesn't clear.
    I'm thinking electrical connection - can anyone suggest what might
    cause these symptoms? What does the engine/electronics try to do at
    4000rpm? EXUP valve?
    Hope the expensive looking black box isn't fried...

    Som, Feb 12, 2004
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  2. Som

    tallbloke Guest

    "Andrewr At Work" <> spouted the
    following in @mygate.mailgate.org:
    Fifty quid for a shagged black box, bite his hand off.

    Could it be a fuel starvation problem? Is air getting intothe tank cap
    to allow fuel out? Try setting the tap to 'prime' and pulling the fuel
    line off the carb to check there is plenty of flow.

    tallbloke, Feb 12, 2004
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  3. Som

    tallbloke Guest

    Bikes from the fifties have off-on-reserve.

    Bikes from the nineties have vacuum tanks which have a prime setting to
    allow the free flow of fuel for cold starting. Or most opf the ones I've
    played with have anyway. YMMV
    tallbloke, Feb 12, 2004
  4. Som

    Champ Guest

    That wouldn't cause the "brick wall" effect at 4k, surely.

    What we didn't ask is if it'll rev past 4k in nuetral
    Champ, Feb 12, 2004
  5. Som

    Champ Guest

    Gravity fuel feed bikes tend to have a prime. Bikes with fuel pumps
    have gone back to on|off|reserve.
    Champ, Feb 12, 2004
  6. Som

    tallbloke Guest

    "Andrewr At Work" <> spouted the
    following in
    My Bike from the 40's has off-on and push. ;o)
    Hmm. On reflection, it may be more an '80's thing. Suzuki and Kawasaki
    used to have 'prime' positions on most middleweights I'm pretty sure.

    Anyway, for the benefit of the OP just open the fuel cap and pull the
    fuel line off to check that there is adequate fuel flow.
    tallbloke, Feb 12, 2004
  7. Som

    tallbloke Guest

    Champ <> spouted the following in

    Ahh, of course. Thanks for the clarification.
    tallbloke, Feb 12, 2004
  8. Som

    tallbloke Guest

    Or if it happens immediately from start up,or with the choke on.
    Or when the air filter was last decrudded.
    tallbloke, Feb 12, 2004
  9. Som

    Som Guest

    Don't think it's fuel starvation - it revs freely to (all of) 3800rpm
    or so, just at 4000 it's like hitting a rev limiter. Won't go beyond
    it. Went for 20 miles to work at 55mph max like that. Really looking
    forward to the journey home.
    Something must be trying to happen at 4000rpm but ain't working?
    Som, Feb 12, 2004

  10. I test rode a YZF750 once and it did exactly the same thing half way
    though my test ride. When I got back to the dealer he said it was the
    exup valve had got stuck.

    I've had the exup valve stick on my old R1 but I dont remember it
    doing this - I remember it wasnt such an abrupt cut off in power.

    Anyway, its the first thing to check I guess.

    Steve Bullimore, Feb 12, 2004
  11. Som

    HedgeHog Guest

    And the YZF is different again.

    It's gravity fed, up to the point when the reserve tank is needed, at which
    point the pump is used to empty the bit below the level of the carbs.
    However the pump runs for a short time to prime the carbs when the ignition
    is switched on.

    And it has no fuel tap at all.

    HedgeHog AKA Paul Hendrick
    YZF750R Bultaco Sherpa
    BONY#3 BOTAFOT#101

    Remove KHH to reply by email
    HedgeHog, Feb 12, 2004
  12. Som

    HedgeHog Guest

    I've had the EXUP valve cable snap on mine, and it would rev to about 5 and
    no more. It also got very noisy around the header pipes when you tried to
    rev it hard.

    I did manage about 70 on it, but no more. The more throttle you opened the
    slower it went, as you're creating even more back pressure.

    Check this first. If it's fuel it'll tend to go alright for a short (maybe
    very short) time, and then run out of puff.

    HedgeHog AKA Paul Hendrick
    YZF750R Bultaco Sherpa
    BONY#3 BOTAFOT#101

    Remove KHH to reply by email
    HedgeHog, Feb 12, 2004
  13. Som

    Muck Guest

    Sounds like an electrical problem to me. Maybe the CDI is fecked?
    Muck, Feb 12, 2004
  14. Som

    Som Guest

    It was the EXUP valve! Seized solid. Few gentle persuasions with WD40
    and a hammer and the revs are back. I didn't think the electronics
    were that complex - must be a sensor telling it to cut the ignition
    above 4000rpm if the valve doesn't open?
    Thanks for the advice and offers of money, it was quite tempting -
    couldn't face another commute at 50mph.

    Som, Feb 12, 2004
  15. Som

    Muck Guest

    Nice one...

    I won't challenge you to a race with my CG125 now though. ;)
    Muck, Feb 12, 2004
  16. Som

    tallbloke Guest

    tallbloke, Feb 12, 2004
  17. Som

    AndrewR Guest

    OK then, 60 quid.

    AndrewR, D.Bot (Celeritas)
    Kawasaki ZX-6R J1
    BOTAFOT#2,ITJWTFO#6,UKRMRM#1/13a,MCT#1,DFV#2,SKoGA#0 (and KotL)
    BotToS#5,SBS#25,IbW#34, TEAR#3 (and KotL), DS#5, Keeper of the TFSTR#
    The speccy Geordie twat.
    AndrewR, Feb 12, 2004
  18. Som

    Som Guest

    Back pressure, that makes sense. Not a sensor. Mmmm.

    Didn't get overtaken by smug looking twats in body-kitted Fiesta this
    morning. Much better.
    Som, Feb 13, 2004
  19. Som

    sweller Guest

    ....but only ones with vacuum operated taps.
    sweller, Feb 14, 2004
  20. Som

    Ben Guest

    This is all rather quaint really.

    Half of the bikes I've owned have had fuel injection.
    Ben, Feb 14, 2004
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