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What size jets for my new xs650f?

Discussion in 'The Garage' started by cafetools, Feb 11, 2019.

  1. cafetools

    cafetools XS650 Addict

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    1978, 17k miles, original motor is healthy, good compression. I cleaned the carbs, had to buy new brass floats (China). I got a carb rebuild kit with a new bowl gasket, new float spring pins.

    I am running UNI filters with red prefilters (oil sticky sprayed) and using Norton commando mufflers. These pipes are loud, you can see straight through them. I think it sounds good though!

    I am using iridium NGK's, and fuel filters. 91 octane premium gas, and a electrosport solid state reg rec.

    So with aftermarket intake and exhaust maybe I am getting a little more backfiring than desired. Maybe less power. If I were to go on eBay (or other site) and buy some jets what should I buy?
     

    Attached Files:

  2. timbeck

    timbeck XS650 Junkie XS650.com Supporter

    Is it a 78-79 ??
     
  3. 5twins

    5twins XS650 Guru Top Contributor

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    With a stock '78-'79 carb set (make sure they are those carbs), the usual routine for mods like yours (the typical pods and pipes) is 1 or 2 up on the pilots and 2 or 3 up on the mains, AND you must lean the needle setting one step with the larger mains. Stock jet sizes for the '78-'79 carbs were a 27.5 pilot and a 135 main. That means you'll want #30 pilots and 140 mains to start. Eventually you could try 32.5 pilots if you feel the idle circuit isn't quite right, and 142.5 mains if you feel it needs it.

    I prefer to use genuine Mikuni jets. They con't cost much, if any, more than the knock-offs and you can be assured of the quality and that they will flow correctly. A carb jet is a precision part. Many have been getting their jets lately from Niche Cycle .....

    http://www.nichecycle.com/ncs/categories/carburetor/mikuni-parts-by-category/jettting-tuning.html

    You will want the N102.221 main jets (called a "small round main") and the VM22/210 pilot jets.
     
    lakeview likes this.
  4. cafetools

    cafetools XS650 Addict

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    It's a 79f standard. Not a special.
     
  5. cafetools

    cafetools XS650 Addict

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    5twins, thanks for the invaluable wealth of knowledge you shared. Hopefully it will run a little better!
     
  6. timbeck

    timbeck XS650 Junkie XS650.com Supporter

    Follow 5T's advice. I run 140 mains and 32.5 pilots.with UNI's and modified exhaust and it works good for me.
    tim
     
  7. XSLeo

    XSLeo XS650 Guru Top Contributor

    When you bought the carb kits, did it come with any internal parts? More than just gaskets. If it did they probably are not the right ones.
    Mikuni built carbs similar to these that fit many bikes. Most carb kits come with generic internal parts. These parts may work one something, just not our XS650's.
    If you have read the carb guide, you know the routine for tear down, cleaning, inspection. You need to do this so you know just what to buy. If stock parts are good, no need to replace. Often if careful you can reuse gaskets.
    Ok, We can assume you did a good job of cleaning. The stock jets in a 78/79 carb set were 135 mains with 27.5 pilots.
    So as 5twin's recommends get a set of 137.5 and 140 mains, 30 and 32.5 pilots.
    I, as others would rather see you use genuine Mikuni parts, better quality and about the same price as Chinese knockoff's.
    If you haven't read the carb guide you can find it in the TECH section.
    I just can't recall the link hook you up.
    Leo
     
  8. 5twins

    5twins XS650 Guru Top Contributor

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    Honestly, I think you can skip right over the one size larger main (137.5) and go right to a 140. If you wanted to get an assortment of sizes, I'd recommend the 142.5 over the 137.5. You might need and use that, but I doubt you ever would use the 137.5. Sure, maybe for testing, for a short period of time, but that would just show you it's too small and you'd be changing it up to the 140.
     
  9. cafetools

    cafetools XS650 Addict

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    Ordered genuine 30s and 140s from niche. Also a gas cap with keys for $20 (which I need badly) hopfully quality is decent.
     
  10. cafetools

    cafetools XS650 Addict

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    5twins, what do you mean about leaning the needle setting? How do I do this? My bike will only stay running with the choke on, and when it's choke is on it boggs down when I give it throttle. I have had the carbs apart multiple times, will changing the needle height fix these issues?

     
  11. 5twins

    5twins XS650 Guru Top Contributor

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    Your needle has an e-clip fitted to the top in one of 5 slots. Where you fit the e-clip allows you to "tune" the needle setting, making it sit higher or lower in the slide. This changes the amount of needle that hangs out of the bottom of the slide and sticks into the needle jet. The amount it sticks into the jet will control how much fuel it lets through .....

    [​IMG]

    The stock position for the e-clip is in the middle or #3 slot. It should be left there with stock jetting but larger mains sometimes require that you move it to a leaner setting. The mains also influence the upper midrange. Larger ones can make this area too rich and leaning the needle setting is the "fix" for that. To do it, you raise the clip a step (to slot #2) which lowers the needle in the slide. Then it sticks down into the needle jet more and lets less fuel through .....

    [​IMG]

    But your problem may be related to how you assembled and installed the needle. There are some spacer washers and a little spring on the needle. They have to be in the right places for the needle to sit at the proper height in the slide. Here's a diagram .....

    [​IMG]

    The thick plastic spacer ring should be above the e-clip, the thin metal washer below it, and the little spring goes on under all that stuff .....

    [​IMG]
     
    TwoManyXS1Bs likes this.
  12. cafetools

    cafetools XS650 Addict

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    So I ordered the wrong jets?


     
  13. 650Skull

    650Skull Dinosaur Lives Top Contributor

    Look for the difference on the Jets you have/got
    web c copy.jpg
     
  14. 5twins

    5twins XS650 Guru Top Contributor

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    You didn't necessarily order the wrong jets. The pilots may be the wrong type though. You need the VM22/210 type for your carb set, the kind with the tiny metering orifice at the top by the screwdriver slot and the larger opening on the bottom .....

    [​IMG]

    As mentioned before, having the small air bleed holes down the sides of the pilot jets doesn't matter. Either style will work in your carbs. But I don't think you can get the no-hole style anymore. I think Yamaha contracted with Mikuni to have them made special while they were producing the bikes back in the late '70s. Today, any VM22/210 pilot jet you buy will have the air bleed holes, even ones bought from Yamaha. If you do want to use the no-hole version, you can scrounge #30's from old XS500 carbs.
     
    TwoManyXS1Bs likes this.

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