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Help Gsxr250 gj72 no power

Discussion in 'Suzuki 250cc In-line 4's' started by Fastfishes, Sep 8, 2021.

  1. Fastfishes

    Fastfishes Member

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    Hello all, I recently bought a 1985 GSXR 250 GJ72 its very tidy and revs freely to 18k in first and neutral but every gear you go up it wants to rev less, so 5th it struggles to rev past 13k. I suspect a carb problem, on looking it has home made air filter, it does not seal at all Does anybody who has this bike know if it will still run OK without an air filter or with one that fits badly ad I know some bikes are air filter critical


    20210902_180418.jpg
     
  2. GreyImport

    GreyImport Administrator Staff Member The Chief Contributing Member

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    Do u mean the airbox is "home made" or just the filter inside?

    If its just the filter then it may be wrong and too restrictive to the air flow .... try it without it
     
    Last edited: Sep 8, 2021
  3. Fastfishes

    Fastfishes Member

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    Airbox is original

    Deffo not too restrictive as big gaps around edges
     
  4. GreyImport

    GreyImport Administrator Staff Member The Chief Contributing Member

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    The airbox controls the ebb and flow of air into the carbys .... if theres a lump of uneven mass in there of unknown density, then that possibly will upset that dynamic , especially at high RPM

    Ride it out on the street so its under load (as opposed to revving the crap out of it in the garage) with an empty airbox
     
  5. GeorgeT

    GeorgeT Member

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    How about checking the ignition timing, CDI or TCi unit, or pick-up coil. Could be not advancing the spark or the wrong duration of the spark at high revs. Just a thought before pulling those carbies apart. I don't know much about 250 GSXR but have had a couple of 750s and the carbies were quite consistently ok. I guess the other variable here is that the revs are unobtainable under load. Could that be a compression issue?
     
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  6. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    Do a compression test.
     
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  7. Fastfishes

    Fastfishes Member

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    Pretty sure its not ignition related, as it revs in neutral and in first, but revs lower in every gear, obv the higher the gear the more fuel is req,and now discovered it revs better on slight choke too
     
  8. GeorgeT

    GeorgeT Member

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    So revving lower under load and requires some choke. Sounds like it may be sucking air somewhere between the carbies and the cylinder causing it to run lean

    So running lean on at least one cylinder. What about an air leak between a carburetor and the head inlet? Spray it with WD40 or starting assist spray when running. If it surges it is sucking in air. Are the carburetors completely separate or do they share a float chamber? I don't know the 250 but had a couple of GSXR750s and they had 4 separate carbies but yours, is I believe a twin isn't it?
     
  9. DannoXYZ

    DannoXYZ Well-Known Member

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    Also check cam-timing and valve clearances.

    Carbs may also refurb and restoration back to factory-fresh clean condition. Meaning complete disassembly down to every last nut, bolt and individual component. Everything scrubbed, ultrasonic soaked and micro soda-blasted.

    Re-assembled with all-new rubbers: fuel-rail O-rings, slide diaphragms, float-valves, and float bowl seals.
     
  10. Fastfishes

    Fastfishes Member

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    Yes Twin carbs with x4 venturis Rubbers all look fine but it is a possibility to explore, will have a look tonight and let you know
     
  11. Fastfishes

    Fastfishes Member

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    All carb rubbers fine. Stripped and cleaned my spare carbs, but does anybody know why inner 2 carbs seem to have more restriction to air flow than outer 2 when you blow from bottom of main jets?
     
  12. DannoXYZ

    DannoXYZ Well-Known Member

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    because secret hidden passages carb body hasn't been thoroughly scrubbed and cleaned.

    Think of carbs like garden hose with sprayer nozzle. Everyone thinks jets! Jets!! Jets!!! Well, you can clean that sprayer all you want, but until you deal with those 29 golf balls stuck inside hose, things just aren't going to flow well.

    it took me over 40-hrs once to properly clean and restore set of carbs for twins.
     
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  13. DannoXYZ

    DannoXYZ Well-Known Member

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    Here's a little more info on restoring carbs to factory-fresh clean condition. Note that spray carb-cleaners no longer work due to removal of chlorinated compounds. Also “cleaning” can mean different things to different people. Last house-cleaning outfit I used didn’t think that “cleaning kitchen” meant restoring top of fridge to showroom condition or pulling out all appliances and cabinets to scrub floor underneath. You’ll want to disassemble carbs down to every last nut, bolt and individual component.

    [​IMG]

    Here’s link to passages that needs to be scrubbed from end-to-end (different bike, however all CV carbs are similar):
    http://n4mwd.blogspot.com/2013/10/secret-ninja-250-carburetor-passages.html

    1. Scrub out all hidden secret fuel-circuits beyond jets with brushes. Amazon - nylon brushes. I like to use 75/25% acetone mix with PEA-based fuel-system cleaner such as Chevron Techron, Red Line SI-1, etc.

    2. Soak in ultrasonic cleaner for hours using polar solvent (PineSol, Totally Awesome, vinegar, etc.) Be aware that Simple Green dissolves aluminium, so avoid it. Amazon - 4L Ultrasonic cleaner

    3. Scrub again with brushes. Pay particular attention to holes where fuel-circuits exit into carb-venturi (neon green dots in photos). Poke those out with soft copper wire. Remove deposits but be careful not to remove any metal as size of these holes are critical to fuel-metering

    4. Be sure to completely disassemble emulsion tube and poke out all lateral bleed holes with copper wire. Same with all jets and bleed holes in carb venturi. Soaking and scrubbing won’t clean these holes and you’ll find wire pushes out dried plastic petrol plugs similar to grains of sand.
    [​IMG]

    5. micro soda-blast all circuits and bleed holes to clean out debris loosened and left by scrubbing. Use finest soda available AircraftSpruce - mini soda blaster

    6. replace all rubber parts: float-valves, O-rings, seals and even slide diaphragms as needed.

    7. adjust float-levels

    8. sync carbs with manometers. While old-style analogue gauges work, I prefer modern digital versions. carb-sync gauges

    Once you’ve done this, carbs will be factory-fresh clean and bike will run like brand-new.
     
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  14. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    In other words, it makes only enough power to do that. Do a compression test first.
     
  15. Fastfishes

    Fastfishes Member

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    Deffo not a power issue as won't really rev any higher even going down hill starts on the button and runs smooth. I'm pretty sure it's a carb issue, but can't get new ones and parts seem like unicorn horns
     
  16. Fastfishes

    Fastfishes Member

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    Problem is getting emulsion tubes out !!! BSW Mikuni carbs I think
     

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