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Carb jetting

Discussion in 'The Garage' started by dlabkeeg, Dec 29, 2009.

  1. gabecustom

    gabecustom XS650 Enthusiast

    you could be right.. I'm no expert, and i haven't seen a flow bench on it, but that's where you'd see any volume changes from one filter to the next, stock or not. foam vs mesh, density of filter etc. all play a part in how the air flows.
    correct me if i'm wrong.. i'm just saying from the way i understand it. :shrug:
     
  2. The previous owner did put aftermarket headers and mufflers, but the airboxes are stock. I'm considering getting UNI pods. Any feedback on that? good idea? not worth it? Also, I read through this thread and have gathered that 1 or two sizes up on the pilots is about right to start? I don't want to go spending a bunch of money for jets that I don't need.
     
  3. gabecustom

    gabecustom XS650 Enthusiast

    From what I've read from other members here the UNI's are the way to go. Going off of what I've read, the K&N style Pleated pod filters create turbulence entering the carb that makes them harder to tune and some what temperamental. Something with the air jet not getting a steady flow of consistent ambient air. The UNI's on the other hand, with their foam filter, have smooth air flow making tuning easier and more consistent.
    So in my mind, UNI's all the way!
    I hope i could help, other wise check out the treads for your self. Just do a Quick search "pods". :thumbsup: good luck
     
  4. Yeah, my brother actually just got an old honda with UNIs, I think I might see what I can get and then try re jet... thanks Gabe
     
  5. How does jet size work? I'm trying to figure out what one or two size up is, but all I see is a bunch of Greek mixed with numbers. What number is one size up from stock pilots on my '75 B?
     
  6. gabecustom

    gabecustom XS650 Enthusiast

    Take a look at the carb charts (from earlyer in this thread) and establish what kind of carbs you have first. Just because you have a 75 year bike doesn't mean you have that year of carbs on your bike. Knowing what carbs you have you can find the stock jet sizes, compare with the sizes that you are seeing to establish they are stock sizes. These bikes have been around for 30+ years and who knows who did what to them.

    When looking for the jet size, only look at the numbers, the number is the size. The "geek" you are seeing is most likely the manufacture's stamp.

    As fare as the "one size up" thing, i'm going to have to ask the same question from others here on the foam.. :bow2: I may know the answer and not know it. :doh: I know jets come in .2, .5, and 1+ etc.. increments, for example, 135.2, 135.5, and 136.
    Is "up one" jet size the first point scale (135-135.2) or the full one size up (135-136)??

    I'm sorry, i really should know this... :doh:
    hope i could help
     
  7. Yeah, that's a start for me, though I do still have the same question as you about "up one." I did verify that they are stock jets last time I had them off. Now to figure out what to buy... :doh: It never ends, does it?
     
  8. Is the higher the number, the richer it runs (larger orifice?) So my #42.5 should move up to #45?


    Actually, now that I'm looking in the book, it says XS650B should have BS38s which has a "4.5" pilot jet and 127.5 main. Now I'm confused.

    I'm sure this has been discussed before, but I can't seem to find the explanation.
     
  9. weekendrider

    weekendrider Iron Horse cowboy Top Contributor

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    The "steps" are figured in 2.5 increments.
    42.5, 45, 47.5, 50 etc.
    The larger the number the larger the jet.
     
    gggGary likes this.
  10. XSLeo

    XSLeo XS650 Guru Top Contributor

    Your 75 should have 45 not 4.5 pilots.
    The best way to find out whast jets you need is to ask your bike. By that I mean try a one up, test it. see if it helps. If it does try another size up. On the pilots usually one up is enough.
    In the carb guide there is asection on how to test carb tuning. Follow the steps.
    As far as spending a ton of cash on jets, that's hard to do. Jets can be bought for about $7 a pair. So buying a set of one up and two up pilots is $14, A set of one, two and three up mains is $21. I bet you spent more than that for beer last week.
    Leo
     
    gggGary likes this.
  11. Very good, thanks guys. Ordered two sets. I just felt like, with my luck, I could easily buy the wrong ones three times. As a recent college graduate, that money is seen in terms of meals. :doh:
     
  12. gabecustom

    gabecustom XS650 Enthusiast

    gggGary, thanks for the c-clamp idea for the float pins! :thumbsup: made one kinda likes your's and it worked like a charm! you da man!!!
     
    gggGary likes this.
  13. gabecustom

    gabecustom XS650 Enthusiast

    Thanks for clearing the "one up" thing for us Weekender! I feel like i need things supper dummy proof some times! :banghead:
     
    gggGary likes this.
  14. So, my problems continue. My bike had 42.5's and an aftermarket exhaust on it when I bought it. When I closed the throttle to decelerate, I got cracking and popping from the exhaust. I thought the bike was running to rich, as indicated by the plugs, but was told those symtoms say too lean on the pilot jets. So, I put 45s in and it made it worse. The bike hardly idles and will not idle at all unless I open the throttles. Spark plugs still come out black. Is it possible that I need smaller jets than 42.5? Could the original symptoms tell me that my valves aren't seating properly and need to be lapped? Something else?
     
  15. weekendrider

    weekendrider Iron Horse cowboy Top Contributor

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    When you went up on the pilot did you adjust the slow jet mix?
     
    gggGary likes this.
  16. Yes, I did. I started at 3/4 out and went both in and out up to three turns. No better, any way.
     
  17. weekendrider

    weekendrider Iron Horse cowboy Top Contributor

    5,686
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    Yes that is the mix screw. If you aren't getting any response from it you will need to figure out why before anything else.
    Are your rubber pj plugs in place? Never mind the pj plugs if you are working with the stock carbs.
     
    gggGary likes this.
  18. It changes, but not significantly. Not enough for it to run well. Not sure about the plug. Where should it be?
     
  19. Okay, didn't see that last part. They are stock
     
  20. 5twins

    5twins XS650 Guru Top Contributor

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    These bikes get changed around so much that before diving into carb jetting, you need to verify which year carb set you're dealing with. Different years used different size mains and pilots, also different type pilots. If your set is indeed the original from '75, it would use a 42.5 BS30/96 type pilot as stock. The other type pilot used, the VM22/210, looks the same externally but is different internally. It will screw into your float bowl but won't flow correctly. Here's how to tell the 2 types apart .....

    [​IMG]

    The other thing to verify would be that the float bowls haven't been swapped for newer ones that use the VM22/210 type pilots. Here's how to I.D. them, by how the air is fed to the pilot jet .....

    [​IMG]

    If these things check out OK, I would next carefully check the float levels and inspect the needle assembly in the slide. Make sure the needle is installed correctly and that it's adjustment clip is in the right spot .....

    [​IMG]
     

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