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Help Acceleration Issue - 1990 ZXR250R

Discussion in 'Kawasaki 250cc In-line 4's' started by Sweeks, Aug 28, 2021.

  1. Sweeks

    Sweeks Active Member Premium Member

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    Hello from the U.S.
    I have a 1990 ZXR250R (Sports Production)
    Having what I think is a problem that some others have had.
    Gone through the carbs, new needles, all o-rings, rebuild petcock, using stock airbox, carbs set to factory specs, fuel levels checked and correct,
    synchronized carbs. Starts, idles, revs perfectly smooth.
    Problem is when I take off it goes nice until about 7000 rpm, then big bog, can hold full throttle for a time, then it takes off. Shift to next gear, then same thing.
    Orings to tees are new, and connected to the airbox.
    Not very familiar with the CV carbs, but it’s so consistent that I believe something is up with the slides/diaphragms.
    Anyone have the answer?
    Thanks
    upload_2021-8-27_11-43-21.jpeg
     
  2. kiffsta

    kiffsta Administrator Staff Member Contributing Member Dirty Wheel Club

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    I had the same issue , it was the orings on the fuel inlet t piece , but you state you have done them
     
  3. Sweeks

    Sweeks Active Member Premium Member

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    Yes, new o-rings
     
  4. kiffsta

    kiffsta Administrator Staff Member Contributing Member Dirty Wheel Club

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    Are they sealing correctly , did it do it before you rebuilt the carbs ?
     
  5. Murdo

    Murdo The Good Doctor Staff Member Contributing Member Ride and Events Crew

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    Try a different TCI as some have a 'funny' dip in the ignition curve around that revs.
     
  6. Sweeks

    Sweeks Active Member Premium Member

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    It wasn’t running when I got it.
     
  7. Linkin

    Linkin The Mechanic Premium Member Contributing Member Dirty Wheel Club

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    What exhaust have you got on it? An open exhaust will make the flat spot worse.
     
  8. Sweeks

    Sweeks Active Member Premium Member

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    It has a Tsukigi Racing exhaust system.
    Has baffles, very quite, not open.
    This is not merely a “flat spot”, it is very dramatic.
    Almost like hitting a wall. A few times it didn’t pick up with full throttle.
     
  9. GreyImport

    GreyImport Administrator Staff Member The Chief Contributing Member

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    Fuel tank breather working?

    Plug leads in the correct sequence?

    Does your model have the vacuum setup to the carbs and fuel tap? .... is it plumbed correctly?
     
  10. Frankster

    Frankster "Play stupid games, win stupid prizes" Staff Member Premium Member Ride and Events Crew

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    This is not merely a “flat spot”, it is very dramatic. Almost like hitting a wall. A few times it didn’t pick up with full throttle

    The ZXR carbs have a fuel bowl vent that normally gets routed into the airbox via an inlet at the top of the airbox. Can you take a pick of your carb setup with the airbox on and post it please.
     
  11. Sweeks

    Sweeks Active Member Premium Member

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    1- I thought about that (has new cap, NOS) I rode it with cap open with no change.
    2- the wires were numbered and put back they way were
    3- #2&3 are connected to each other, #1&4 connected to tee then to fuel tap, which has new diaphragm and seals.
     
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  12. Sweeks

    Sweeks Active Member Premium Member

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    I will go get a pic shortly.
    But, it has 2 tees between the 2 pairs of carbs, that connects to 1tee, then hose goes over air box lid to the nipple at top rear of airbox.
    Someone suggested that I recheck the air jets at the diaphragm vent,
    I am sure that they are there, I suppose I can recheck, if one were missing or clogged; would it cause this problem?
     
  13. Sweeks

    Sweeks Active Member Premium Member

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    7953E2C3-AC09-4EB8-A955-5BD9C6B0A383.jpeg CD686B37-49E4-4BCC-84B3-621B3A5D6658.jpeg
     
  14. TonyZXR

    TonyZXR Well-Known Member Premium Member

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    No one has said it yet but this should have the 32mm carbs being that is the B version . Wasn't this version notorious for having that bad flatspot?
     
  15. Sweeks

    Sweeks Active Member Premium Member

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    Yes, 32mm.
    I would dare to say that this can’t be normal.
    I thought that I have seen some other posts where someone had this problem.
    I don’t have any experiences with these CV’s.
    Was hoping that it was a known common problem
     
  16. ShaneP

    ShaneP Well-Known Member Premium Member

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    Does it bog or cut out? Is it rich or lean? Make sure your slides are operating smoothly, not sticking, Venturi hole isn't blocked. If it is ok or persists, then it might be a carburetion issue, and the needle needs adjusting (easier said than done if you have to lean it out). They don't run as well without the airbox, but try it so you can see if the slides are operating properly and you might be able to test some things like covering the intake a little to richen it
     
  17. Sweeks

    Sweeks Active Member Premium Member

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    My Bike:
    1990 Kawasaki ZXR250R, 1984 KR250, 1973 Z1, 1972 H2, 1973 H1, 1973 Bighorn
    It Bogs, pretty sure it’s lean.
    Otherwise it would foul the plugs.
    Slides are good, no sticking.
    It revs smooth and seems perfect with the airbox on or off while sitting still. Takes off as if things are good.
    Somewhere around 7k rpm, it just falls on its face. If I open the throttle at that time, it continues the bog but after a time, it revs again. Shift, then same.
    It does not miss or skip.
    I think that the slides are closing due to lack of vacuum, but that’s just a guess. I have rebuilt plenty of carbs, but these are the first CV’s that I have done. They are clean, and have been set up exactly to the service manual specs. I am planning to recheck the fuel levels in the bowls again and someone suggested that I check the air jets at the diaphragms.
     
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  18. ShaneP

    ShaneP Well-Known Member Premium Member

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    CV is constant velocity, so the slides are operated by air speed past them (the hole on the downside of the slide is the main opening for lifting the slide, but check the other air channels and jets). If you think it is lean, put a small o-ring under the needle to increase the mid range fuel. But you could test by pulling the choke on to see if that improves it when it bogs down. As @Murdo said, could be an TCI/ecu issue, or as mentioned being a B model, the carby settings may be different - someone may have the wrong needles in it. Good luck
     
  19. Frankster

    Frankster "Play stupid games, win stupid prizes" Staff Member Premium Member Ride and Events Crew

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    CV bowl vents seem correctly plumbed. Are we 100% certain that this is an SP ZXR250? If yes, is it a B1 or B2?

    ZXR250B1
    Frame: ZX250A-300001-301500
    Engine: ZX250AE000001-
    TCI: 21119-1263

    ZXR250B2
    Frame: ZX250A-301501-
    Engine: ZX250AE010001-
    TCI: 21119-1290

    Also...

    What's the code on the carbs?
    What's the number on the needles?
    Main Jet size?
    Pilot Jet size?

    You seem to know what you're doing, so I'm just chasing these details to eliminate any unnecassary checking to fix the problem(s).
     
  20. Linkin

    Linkin The Mechanic Premium Member Contributing Member Dirty Wheel Club

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    When I had my C model on a dyno at TAFE, at 7k RPM it was rich, at about 10.5:1 - in fact it was rich all over (standard settings with open exhaust) but at 7k RPM it was very rich.

    I know for a fact that air leaks make this problem worse. You have done the T-piece o-rings which is the problem the majority of the time. I suggest you also check the carby boots themselves for air leaks with a solvent while running the bike. Any increase in RPM = leaky. New carb boots are available. They tend to leak around the vacuum ports and the sealing o-rings to the cylinder head compress and dry out.

    Also replace all of the vacuum lines. I would also consider ditching the fresh air valve which I never got around to doing.

    Just to rule it out, I would suggest checking the valve clearances and the condition of the chain / tensioner. A known weak point on these bikes. Sloppy cam timing can cause issues.

    I also spy some debris in #4 around the air jets
     
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