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XS1 carb tuning

Discussion in 'The Garage' started by Cribby, Dec 6, 2019.

  1. Cribby

    Cribby XS650 Enthusiast


    I’ve just had the carbs on my XS1 professionally rebuilt but the bike is running rich as hell

    I use a Gunsons gas tester which works great on all my bikes but this one I can’t get any where near the reading I need. No matter how far in or out I move the air screws, the bike still runs rich


  2. Mailman

    Mailman Hardly a Guru Top Contributor

    Hi Cribby,
    I know you just paid to have this done but unfortunately you are now in the position of not really knowing what’s in your carburetors. 5Twins has correctly pointed out that most rebuild kits available online contain the wrong size jets for your make and year carburetors. Also you have no way of knowing if the float height was correctly set and I’ve bought new gaskets that don’t fit without a lot of trimming. Did your builder install new jets or just clean everything up?
    2M posted about how much is original jets had worn, increasing the size of the jet.

    I myself installed the correct size pilot jets on my XS2, but they didn’t have the aeration holes on the sides like they were supposed to and my bike ran like crap until I figured it out.

    These carbs are so touchy about being properly set up. I just don’t know how you would proceed until you see for yourself what your jetting is and how your floats are set. Gary always like to recommend doing a hot water dunk test on floats to look for bubbles.
    Some guys here are real pros with carbs. But for me, it always seemed , I had to install / remove/ inspect / re install a few times before I got everything working right.
    Good luck, maybe someone more knowledgeable will chime in.
  3. grizld1

    grizld1 Grumpy old man Top Contributor

    Click the Tech button and find the link to the old carb guide. It will give you year-by-year specs and components. +1 on checking floats and fuel level; when there's no response to mixture screw changes, that's the first thing to look at. Avoid kits and knockoff Chiwanese brass; genuine Mikuni parts will save time, aggravation, and money.
    lakeview, Jim and Mailman like this.
  4. gggGary

    gggGary Stop that! XS650.com Supporter Top Contributor

    I'm running that XS1 I stole from under you, :rolleyes: All stock pipes airboxes and new stock jets. All is good.
    credit 5twins 4 images
    CarbSpecs.jpg mikunipilotjetsmoddedam2.gif
    Last edited: Dec 7, 2019
    Mailman, GLJ and Jim like this.
  5. TwoManyXS1Bs

    TwoManyXS1Bs BBQ Hunter Top Contributor

    gggGary likes this.
  6. Cribby

    Cribby XS650 Enthusiast

    Ah, so it was you?
    gggGary likes this.
  7. 650trader

    650trader XS650 Enthusiast

    GENUINE MIKUNI brass cost a fortune. Mikuni sells their carbs with brass that works on nothing
    and then make a fortune selling you what you really need.
  8. Jack

    Jack XS650 Junkie Top Contributor

    There's no such thing as a bolt on and run it aftermarket carburetor ,there are just to many engine variables in question unless you buy from 650 Central where the jetting will be very close. In regards to Mikuni selling carbs with jetting be off, you need to buy from a distributor who will work with you on exchanging the jetting or offer some discount on the jetting.
    Mailman, TwoManyXS1Bs and gggGary like this.
  9. gggGary

    gggGary Stop that! XS650.com Supporter Top Contributor

    Dunno? stock Yamaha Mikunis seemed to work pretty well as jetted. ;)
    I buy genuine mikuni brass, often from Niche supply, not that expensive, I prolly have over a hunnert jets by now. :shrug:
    Last edited: Dec 10, 2019
    Mailman likes this.
  10. arcticXS

    arcticXS XS650 Junkie

    I wonder how much the properties of pump gas has changed over the last 50 years. Surely it has changed enough to require subtle (or not so subtle) changes to carb jetting and tuning. Maybe a step or two up on pilot jets can improve things? At least ethanol mixed into gas effectively leans out the mixture. I once dumped a liter or two of E85 into an over half full tank of Shell V-Power 98RON. And the bike would not run properly without a little bit of choke.
    Team Junk, TwoManyXS1Bs and gggGary like this.
  11. 650trader

    650trader XS650 Enthusiast

    Just to clarify - I was talking about the aftermarket VM.s not the stock CV carbs.
  12. arcticXS

    arcticXS XS650 Junkie

    I believe you need to list ALL jet sizes, including needle jet, needle size and slide cutaway. And carb size (32/34/36/38?)
    Some background info would also be helpful, i.e are the carbs used, or purchased new for your XS, and so on. It is very possible that VM carbs come with a baseline setup for a 2 stroke......
    gggGary likes this.
  13. gggGary

    gggGary Stop that! XS650.com Supporter Top Contributor

    I keep a variety of jets for stock and modded bikes to fit BS38, BS34, and the VM series of Mikuni.
    When I mess with other brands of carbs I'll buy jets for those.
    If you use carbs and want it to run right that's what you do.
    I'm not a great tuner but I get by with a little help from my friends.

    Attached Files:

    TwoManyXS1Bs likes this.

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