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Bs38 CV jetting.

Discussion in 'The Garage' started by koyuga, May 26, 2019.

  1. koyuga

    koyuga XS650 Enthusiast

    Could someone give me an idea on what jets I need.

    I have a pair of unlinked 38s that came with my 1980 special roller.

    The engine has been rephased 277.
    Cam rephased with aggressive street grind
    Coils are the green high output.
    Charging is PMA, ignition pamco.
    Custom header pipes outside diameter 1.5" with peashooters
    Filters are unipods.
    Located at sea level.
    I'm thinking bout getting a selection of mains and pilots.,
    Mains I'm thinking 140/145/150.
    Pilots #30 or 32.5???

    I'm not sure but I think my jets are the hex style.

    Thanks for any replies.
    peanut likes this.
  2. Bushyeyed1

    Bushyeyed1 Cafe Racer XS650.com Supporter Top Contributor


    Lots of good information can be found in the tech section. And even a search can probably bring up a lot of bs38 set ups. For tuning carbs start with main first, second the needle, then pilot. It will make sense once you read the thread I provided.
  3. 5twins

    5twins XS650 Guru Top Contributor

    Your unlinked BS38s would be one of the 2 early sets. Here's a list of the jet specs over the years. As you can see, the early carbs would have a stock main of 127.5 or 130. Usually for mods like yours, 2 to 4 sizes up would probably be what you need. I'm thinking anything above 140 would be too much. You'll probably end up in the mid to upper 130's.


    Your pilots should be the BS30/96 type and sized in the 40's. One or two sizes up on the pilots is usually the norm.


    Your mains are not the hex type but rather what are called the small round type, Mikuni part #N102/221.

    gggGary likes this.
  4. peanut

    peanut XS650 enthusiast & inveterate tinkerer Top Contributor

    it depends on which type of BS38s you have . Are they 76-77 or 78-79 type ?
    Which slides and jet needles do you have ?,do the carbs have the pilot and choke inlets in the mouth of the venturi ?
    You're in the ball park but I would suggest starting with a 35 pilot and going down if necessary to a 32.5 but no lower or your idle and decel mixture will be too lean .
    I currently have 145 main and 35 pilots on my 79 BS38s with the later 78-79 502 jet needles and slides.

    PS just saw 5Ts post . I just assumed that the carbs were later types but with missing parts because of the 1980 model year . It didn't occur to me that someone would try and use really early BS38s on a 1980 :) we really need to confirm ID of the carb model
  5. koyuga

    koyuga XS650 Enthusiast

    Great thank you.
    gggGary likes this.
  6. 650Skull

    650Skull SSSSSSSSSlither Top Contributor

    This Graph by 2M in conjunction with 5Twins graph.
    23306-1446134303-c24091a67909b7d9bb10cccc86fb2cdc Text Trade mark.jpg
    TwoManyXS1Bs and gggGary like this.
  7. gggGary

    gggGary I'm listening, change my mind XS650.com Supporter Top Contributor

    Yeah Peanuts confusion is understandable, not many opt for the early separate carbs given a choice. They are more for the restorer crowd. They can work well but over 10 years the carb industry made some significant advances. Starting from scratch, I hear the ninja carbs work well, there's a thread.

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