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Carb jetting

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

  1. grizld1

    grizld1 Grumpy old man Top Contributor

    The photos seem to show fuel level significantly higher on the left. BTW "grizzled" means "streaked with grey," which described my head back in 2004 (nearly all white now); no intended reference to bears:).
     
  2. 5twins

    5twins XS650 Guru Top Contributor

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    You mentioned having UNI pods but not which style. Do you have the longer, full foam (on the ends too) type like this .....

    [​IMG]

    ..... or the shorter style with the rubber end caps? Personally, I don't care for the shorter ones. They don't flow as much air obviously because they're shorter (smaller) but also because the ends aren't open too. In fact, a local guy installed them and we didn't need to do any jetting changes. That tells me they didn't increase the air flow much, if at all. If you have the short capped ones, maybe you could do some testing with them removed and see if that cures the richness at idle? The type of filter oil you use on these black foam pods can also have an effect. The normal foam filter oil is supposedly too thick and can clog them. The recommended oil is simply a 50-50 mix of gas and motor oil.
     
    grizld1 likes this.
  3. ebb0tk

    ebb0tk XS650 Addict

     
  4. ebb0tk

    ebb0tk XS650 Addict

    You think that would matter? I dont think i can get them any better. Both were almost spot on at the flange, if they are suppose to be there? The pictures could have been taken slightly in an angle too ;D

    Haha, all that gas does make the head kinda grey here aswell :)
     
  5. grizld1

    grizld1 Grumpy old man Top Contributor

    If fuel is level with the flange it's fine; must be the angle of the shot on the right hand carb that fooled me. Affirmative re. longer Uni pods; the more volume, the better. On vacuum carbs you'll get best results from a still air box with free flowing filter elements.
     
  6. ebb0tk

    ebb0tk XS650 Addict

    Had to buy the unis from the states and took some time. I can get a hold of these from ram air (heard good stuff about em) they are still short but no capped ends. Thoughts?

    https://rover.ebay.com/rover/0/0/0?mpre=https://www.ebay.co.uk/ulk/itm/350952699377
     
  7. gggGary

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

    For a trial wash some oil out of what you have, see if it makes a difference. not high on my list of culprits though, I think that would tend to be more of a problem at high RPM when more air is being moved. Looks like you did fine but I'll mention here that even a small air bubble trapped in the clear line will affect the apparent fuel level.
     
    Last edited: May 11, 2018
    grizld1 likes this.
  8. ebb0tk

    ebb0tk XS650 Addict

    I have some new air filters that does not have a cap for trial now.
    But what i did try was, (with the 45 pilots) i adjusted the mixture screw to 3 turns out from bottom. And the bike started to pop alot on decel. Wouldnt that indicate a lean condition? :O went back to 3,5 turns out and no pop on decel.
     
  9. 5twins

    5twins XS650 Guru Top Contributor

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    Yes, the popping would indicate a lean condition, and the fact that it went away when you opened (richened) the mix screws confirms that. On the BS34s, the air jet in the bell mouth that supplies the pilot circuit can also be changed. This has less of an effect than changing a pilot jet so can be used for fine tuning. You can achieve richness levels in between pilot jet sizes. This can come in handy if, for example, a 45 pilot was too lean but a 47.5 size was too rich. By reducing the air jet a size or two with the 45 pilot, you could get a richness level in between the 45 and 47.5, say like a size 46 if there was such a thing.
     
  10. ebb0tk

    ebb0tk XS650 Addict

    Then again i wonder why the mix screws can be turned almost fully in (0.5 turns from bottom) and no noticable sound on rpm drops. At 0.5 turns out it dies.

    For notice, i have not yet tried the dead cylinder with the other air filters (ram air race pods without cap)

    My first thought was pilot was too big, but then again the mix screws indicated lean at 3 turns out.

    Are my needle and needle jet bleeding over too pilot circuit? :eek: need to read the carb guide again :)

    Because i was experiencing a rich pilot at first. It would stumble a bit off throttle.. i

    Ill give those new filters a try and see if they make the mixture screws more sensitive.

    Alot at once here hehe :)
     
  11. 5twins

    5twins XS650 Guru Top Contributor

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    The mix screws on the BS34s are not very responsive because of all the pollution mandates. What you're describing sounds normal. They don't have much effect on running that you can actually hear until they're nearly closed or way far open. But, they are doing stuff. If you were to test it with an exhaust gas analyzer, you would see the changes even though you're not hearing them.

    On the older carbs, you usually hear changes or get engine stumbling once you get just 1/4 to 1/2 turn past the "sweet spot" or best setting in either direction. They're pretty easy to set by ear because of this. As you've discovered, the pollution controlled BS34s don't respond like this. Because of that, when setting the BS34 mix screws, I rely more on little throttle blips to test my setting. If I get lots of popping when doing the throttle blips, I know the screws are set too lean. I keep making them richer by maybe 1/4 turn at a time until the popping goes away. Also, I do both carbs together. On these carbs, having mismatched mix screw settings can cause popping too.

    So, I start with both mix screws set at 3 turns out. Then I start working both of them in together in 1/4 turn steps - both to 2 3/4, both to 2 1/2, etc. - doing throttle blips after each adjustment, until I start to get the popping. Then I work them back out together - both to 2 3/4, both to 3, both to 3 1/4, etc. - until the popping with throttle blips goes away.
     
    Wulfbyte and robinc like this.
  12. ebb0tk

    ebb0tk XS650 Addict

    Thanks 5twins. Makes sense! And as always, good help around here :)
     
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