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Carbs Mikuni BS38 jets.

Discussion in 'The Garage' started by mike-dubai, Sep 16, 2016.

  1. mike-dubai

    mike-dubai XS650 Member

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    dubai
    Hello,
    It would be great if someone could help me to choose new jets before ordering my parts from US. (I live in Dubai UAE!)
    I have a 650 XS special 1978 with Mikuni BS38.
    I added two XS performance Pod Air Filters (http://www.mikesxs.net/parts/yamaha-xs650-57mm-tapered-oval-pod-air-filter-2-1-4)
    And two bigger than stock Stainless Steel XS Performance Headpipe Set - 1.75" (45mm.) OD x 1.65" (42mm).
    (http://www.mikesxs.net/parts/yamaha-xs650-stainless-xs-perf-headpipe-set)
    With two Lossa reverse cone mufflers (http://www.lossaengineering.com/col...-brand-reverse-cone-muffler?variant=856209431)

    Main jet: 135
    Pilot: 27.5
    Don't know if someone has some near conditions in the US (i don't think so!), it could help me to change the jets... The bike is not working well at all like that.

    Average temp 33°C or 91.4°F (in between 25° and 40° degrees),
    Average humidity around 40%.
    Alt: sea level...

    What will you try?

    Thanks,
    MIchel.
     
    Last edited: Sep 16, 2016
  2. peanut

    peanut XS650 enthusiast & inveterate tinkerer XS650.com Supporter Top Contributor

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    You may well find that increasing your pilot jets to 30.0 and adjusting the air/fuel screw will be sufficient . I have foam filters on my 79 BS38's and increasing to 30.0 worked well for mine. Like you I also have non standard exhausts which will almost certainly require larger main jets and adjusting the needle position but this will need to be arrived at by testing .

    My advice would be to buy some pairs of 6mm main jets say 140 145 150 and start with the richest jet 150 and work your way down . You need to tune each throttle section seperately . effects3.gif
     
    Last edited: Sep 16, 2016
  3. 5twins

    5twins XS650 Guru Top Contributor

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    Some of the components you've chosen may make perfect tuning impossible to achieve. Your pleated style pods don't work well on CV type carbs. Foam pods are a better choice. Your 1 3/4" headpipes are bigger than ideal for this engine. You may need to add the inserts at the head .....

    http://www.mikesxs.net/parts/yamaha-xs650-headpipe-torque-inserts-pk-2

    In the sizes we usually use, Mikuni mains and pilots come in 2.5 number increments. I would suggest getting 30 and 32.5 pilots, 140, 142.5, and 145 mains. The 32.5 pilots may be difficult to find as they are a rather new size offering from Mikuni, but they may be just what you need.
     
  4. peanut

    peanut XS650 enthusiast & inveterate tinkerer XS650.com Supporter Top Contributor

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    6mm main jets don't have to be Mikunki jets . They can be any generic 6mm jet with a 8mm overall length.... Delortto ....Polini... lots of different makes will fit and work just as well ;)
    I've not seen any 32.5 pilots I'll keep an eye out.
    edit
    Found these which I believe are the correct type for a 1978/79 BS38 I know that there is also a pilot jet with no air holes in the body but can't find your post 5T mikuni pilot jets.jpg
     
    Last edited: Sep 16, 2016
  5. peanut

    peanut XS650 enthusiast & inveterate tinkerer XS650.com Supporter Top Contributor

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    Mike if you can't get any over there PM me your address and I'll post you a couple of pairs
     
  6. 5twins

    5twins XS650 Guru Top Contributor

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    I use only genuine Mikuni jets. You can be assured of their quality and flow rate. But it's your bike, do what you want.
     
  7. peanut

    peanut XS650 enthusiast & inveterate tinkerer XS650.com Supporter Top Contributor

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    well I never trust anybody so I have sets of jet gauges and a venier guage to check all the jets I use on bikes , scooters and cars.;)
    I was just making the point to the PO that like lots of other things on the XS650 you don't need to buy Yamaha labeled parts Generic oil seals and bearings and other components are equally as good if not better. I think this silly OEM nonsense can be taken too far sometimes
     
  8. 5twins

    5twins XS650 Guru Top Contributor

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    I didn't say buy the jets from Yamaha. They are Mikuni jets of course, but at nearly twice the price because they come in a Yamaha bag. There are many other places to get genuine Mikuni jets at their normal price, sometimes even less. Personally, I buy the 4 packs for 4 cyl. bikes. They're cheaper that way and I do lots of carb tuning so I need the supply .....

    [​IMG]
     
  9. peanut

    peanut XS650 enthusiast & inveterate tinkerer XS650.com Supporter Top Contributor

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    yeah i hear what you are saying 5T but I suppose I see things from a different perspective .

    Here in the UK prices of XS650 bikes are double even triple the price of the USA . The same goes for parts . The USA had a huge number of XS650 and parts so prices are still low enough to be able to afford OEM parts .
    I feel really sorry for all the xs650 owners down under where parts are in very short supply and extremely expensive to import .

    Your advice is good especially for those new to repairingthese old bikes, you should always be able to trust genuine Mikuni parts.
     
  10. mike-dubai

    mike-dubai XS650 Member

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    dubai

    Hello 5twins, thanks for the info. You're not a funny guy!
    I have already the inserts but would like to try without. I don't understand why they buy bigger pipes just to reduce them with the insert....
    I rode already with the pods (and stock exhaust), the bike works fine but not with the new pipes....
    I will buy two or three differents jets and one or two main pilots and try to tune...
    Thank you again, Michel
     
  11. mike-dubai

    mike-dubai XS650 Member

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    Thanks for the info Peanut, i should find what i need...
     
  12. 5twins

    5twins XS650 Guru Top Contributor

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    Bigger diameter pipes flow better down away from the head but these motors like a smaller diameter coming out of the head for the first few inches. Hence the inserts. You "fool" the motor into thinking it has smaller diameter pipes. So, you get good flow out of the head and also down the line away from the head. Larger diameter pipes are supposed to move the power band higher up in the RPM range and also cause some low RPM power loss. A back to back comparison with and without the inserts would be good. That will probably show you this.
     
    mike-dubai likes this.
  13. 5twins

    5twins XS650 Guru Top Contributor

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    CV carbs are very forgiving and can mask over minor jetting glitches. For this reason, when jetting them, you have to work them hard to reveal any potential jetting issues. You need to test the various RPM ranges and the transition areas between circuits using large amounts of throttle, even full throttle sometimes. If you don't test like this, you may not find the glitches.

    When sorting the mains, an important area you test through is the midrange to main transition range. This is from about 4K to 5.5K. As you increase your main jet size, eventually you will start to get break-up through this range under heavy throttle applications. This indicates the bleed-over from the larger mains into the upper midrange is making it too rich. You fix this by leaning the needles a step. You can then continue to increase the mains but you usually can't go much bigger before the break-up returns. This is pretty much the limiting factor for the main jet size on these carbs. Now, leaner needles and/or needle jets could be installed and that would allow even bigger mains, but none are available for these carbs so we're kinda stuck with what we've got.

    CV carbs rely on a smooth flow of air to create vacuum and lift the slide. The pleated style pod disrupts this air flow and that can mess with the slide lift. The slide can actually flutter instead of lifting smoothly. Sustained slide lift on a CV carb doesn't occur until late, like 4K RPMs. This is the same area you'll be watching if trying to sort your mains. The pleated pods can contribute to the break-up there and make it occur sooner than if you had better smoother flowing foam pods. From the break-up you're getting, you may assume your main is too large when it may not be. This was the problem the pleated pods caused on my bike. I felt I needed another size larger on my mains but the upper midrange break-up it caused wouldn't let me run it. I switched to UNI foam pods and was able to pop that larger main in no problem. All the upper midrange break-up had disappeared.
     
    mike-dubai likes this.
  14. mike-dubai

    mike-dubai XS650 Member

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    Wahoo,
    I should have read this before buying parts...
    Anyway thank you very much for your explanations.
    I will let you know the settings or changes...

    Michel.
     

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