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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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    My Bike:
    1990 Kawasaki ZXR250R, 1984 KR250, 1973 Z1, 1972 H2, 1973 H1, 1973 Bighorn
    Yes, 100% B2 SP.
    Frame= ZX250A-301691
    Engine= ZX250AE
    As per manual: main jet-#1&4=135, #2&3=130
    Pilot=35, needle-#1&4=N76A, #2&3=N52X
    Air screw= 2 1/4 turns
    Fuel levels verified with factory float gauge.
    Synchronization with MotionPro SyncPro manometer.
    Again, Starts, Idles perfectly at 2000rpm. Revs as high as you like (no load).
    No sign of vacuum leaks, new vacuum hoses to carb spigots.
    Has original bowl vent hoses to airbox.
    It is completely stock other than the Tsukigi Racing exhaust, 4 into 1 collector, muffler with baffles are quieter than stock.
    I have checked the valve lash settings and they are in spec.
    Upon inspection, it appears that the timing chain, guides, and tensioner have been recently replaced.
    Yes, I saw the bit of trash on #4 carb, but am sure it fell there when I removed the airbox.
    Igniter # is 21119-1290, which is correct from what I have found on Megazip.
     
    Last edited: Aug 30, 2021
  2. Sweeks

    Sweeks Active Member Premium Member

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    I am sure that I would get the best information from a Dyno session.
    No access to a bike dyno, only autos.
    It may sound arrogant, but is NOT meant to be;
    I have restored and tuned many bikes.
    Mostly Kawi 2 strokes, H1, H2, F9, KR.
    And
    Kawi Z1.
    These Keihn’s aren’t seriously complicated to me.
    I cleaned them in my ultrasonic cleaner and made all passages clear.
    All float needles, o-rings, fuel tee o-rings, bowl vent o-rings, airbox-to-carb seals.
    Verified that all jets, needles are correct.
    In this respect, it should be as delivered from the factory.
    Obviously, I have either overlooked something or “some” part is malfunctioning.
    I guess I need to read up on these CVK’s.
    I don’t fully understand the operation of the diaphragm controlled slides.
    Which is why I have guessed that they aren’t opening when they should.
    I haven’t completely ruled out the ignition timing. My normal M.O. is to start with factory settings and parts, get as good as possible, then start tuning improvements, shimming needles etc.
    I appreciate everyone’s input and comments, as the discussion tends to keep us thinking.
    I joined this group because I know that you all have experience with these bikes and I thrive on information that you bring.
     
    Last edited: Aug 30, 2021
  3. Frankster

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

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    HI Sweeks,

    Yes, it sounds like you know what you're doing and have logically gone about trying to resolve your issue. I've not worked on the 32mm Keihins, but they should be the same as the normal 30mm units.

    As you probably know, CV carbs rely on air movement to do 'things'. When you have them off the bike, if you blow air through the section highlighted in RED, the slide should instantly be pulled up and when you stop blowing, the spring should immediately push the slide back into its static position.

    The jets and orifices at the bottom of the carb (in ORANGE) are also affected by airflow. These can sometimes have a blockage which will cause odd things to happen at idle and up to the point where the main jets take over from idle jets. From your description, It sounded like your bike was running out of fuel, which is why I asked about the bowl vents. Having eliminated them from the list, now seems like maybe, under load, the carbs aren't all working properly. Somewhere in all that should be your problem.

    The TCI unit on your bike does have a peak around 7,000rpm, but it should only hesitate. What you're describing sounds far too severe to be totally TCI related. Having said that, I've never had a TCI fail, so I'm not sure what the symptoms would be, so you might want to try and get/borrow another TCI to confirm it's not part or all of the problem. Keep trying, we'll get to the bottom of it sooner or later.

    ZXR Carbs.jpg
     
  4. Frankster

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

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    Out of curiosity, what plugs are you running?
     
  5. TonyZXR

    TonyZXR Well-Known Member Premium Member

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    As I mentioned in the facebook group I still think the 4-1 headers are causing issues, will be interesting to see when you change them back to standard if the problems still there
     
  6. Frankster

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

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    I missed that the bike has an aftermarket exhaust system (4 to 1 headers) . Can you post a pic of it please?
     
    Last edited: Aug 31, 2021
  7. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    Exhausts rarely do anything, IMHO. Carb diaphragms have no pin holes or pinches?
     
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  8. Sweeks

    Sweeks Active Member Premium Member

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    NGK CR8E
     
  9. Sweeks

    Sweeks Active Member Premium Member

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    C0717E90-7034-4A5D-AC2F-126B2AF3234B.jpeg DA28513D-CF5A-47DE-A2A1-48F2D751C9E3.jpeg 7D3455E2-D27B-4BFE-AC9B-CFD19DBEA978.jpeg 8C8DDA03-DE70-49D5-BA02-A7786DEF7A6D.jpeg
     
  10. 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
    I have removed the carbs,
    Removed the tops to recheck the diaphragms and found #1 diaphragm was not completely positioned into the outer recess. Not much, but it could have caused a leak.
    The diaphragms are in very good condition.
    I am going to recheck the fuel levels while they are easy to access.
    Had planned to put the old exhaust back on, but I may test ride it first.
    Problem is, we are experiencing the rough weather from the passing hurricane IDA.
    Will have to wait until the rain stops.
     
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  11. GreyImport

    GreyImport Administrator Staff Member The Chief Contributing Member

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  12. Frankster

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

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    Had planned to put the old exhaust back on, but I may test ride it first. Problem is, we are experiencing the rough weather from the passing hurricane IDA. Will have to wait until the rain stops.

    Yes, that's one tasty Hurricane.

    The diaphragm sitting right should help. Is the collector on your exhaust headers the standard one or has it been cut off and replaced? The image is a bit dark to see the detail. But, as @maelstrom said, a different muffler won't make any real difference other than noise. Most of the action has happened by the time the exhaust gasses and pressure waves have hit the collector.
     
  13. Sweeks

    Sweeks Active Member Premium Member

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    554ADC2A-F89A-44B9-B577-5C3F3C14E774.jpeg
     
  14. 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
    All new exhaust, was just replaced before I purchased the bike.
    Fortunately, I got the old exhaust with the deal.
    I just rechecked the float (fuel) levels, perfect.
    I have done some reading and have a much better understanding of the CV’s.
    As the diaphragm/slides are raised by vacuum, I believe #1 was more than likely, leaking.
    If I replace the exhaust at this point; I will not know which change made the difference, if any.
    So, I will ride it to see if seating of the diaphragm made a difference.
    Then I will install the original exhaust, then ride again.
    I found a used igniter on eBay in Australia. It’s on the way, wouldn’t hurt to have a spare.
     
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  15. Sweeks

    Sweeks Active Member Premium Member

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    Update:
    Test rode with carbs rechecked, same problem.
    Re-installed original exhaust system.
    Seems the same problem, but maybe at slightly lower rpm.
    It revs good, while on the stand. Even though I can hear a little stumble around 4-6k rpm.
    When riding, at very little throttle, it seems smooth. When giving more throttle, it bogs and acts as if it were running out of fuel. Sometimes at wide open throttle, it pushes through the large bog, then takes off.
    Even though it is a SP version, it really isn’t rideable like this.
    I have an igniter on the way but I don’t believe it is an ignition timing issue.
    Only other thing that isn’t stock, I’m running 110 octane race fuel.
    I guess it’s worth a try to put pump fuel in it.
     
  16. Linkin

    Linkin The Mechanic Premium Member Contributing Member Dirty Wheel Club

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    The fuel might be the issue. A lot of high octane fuels are designed for really high compression ratios and larger, lower rpm engines compared to these small screamers. Fast fuel burn is critical

    Here in AUS I only ever used 91 octane without ethanol. I would suggest trying a tank of the lowest octane you can buy without ethanol.

    It is common here to have to rejet carbed bikes for our 98 octane as the density of the fuel is higher than the lower octane stuff. Without doing so you end up with incomplete burn and lean condition in the cylinder, rich condition in the exhaust and lots of carbon out the exhaust.

    A few of the 250 fours I've had have not liked our 95 or 98 octane, and run best on 91 octane. I'm not sure on what your equivalent is as we use different ways for specifying octane.
     
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  17. Frankster

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

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    Only other thing that isn’t stock, I’m running 110 octane race fuel.

    Run any pump octane fuel and CR9E plugs gapped at 0.7mm
     
    Last edited: Sep 8, 2021
  18. GreyImport

    GreyImport Administrator Staff Member The Chief Contributing Member

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  19. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    USA uses Anti-Knock Index which is an average of MON and RON. Aus uses RON. So 91 Aus is like 87 USA
     
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  20. Sweeks

    Sweeks Active Member Premium Member

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    Well… changed to 87 octane with no improvement.
    Next up, I will try the igniter when it arrives.
    The 32mm carbs are hard to find.
    I purchased a set of 30mm for a regular A.
    Just ordered a seal kit from Litetek.
    Looking for some replacement float needles.
    I will build the 30’s and try them just to see what changes.
    That should help me narrow it down.
     

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