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Carburetor adjustment

Discussion in 'XJ Technical Chat' started by Isaac G, Jul 15, 2019.

  1. Isaac G

    Isaac G New Member

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    my 85 maxim xj700 needs a few adjustments. I pulled my plugs and they are black and sooty. Also took the time to pull my carbs and do a deep cleaning on them. Floats are correct height, no overflow of fuel coming from them. Idle screw is set for 1 on the tachometer. Any idea how many turns, from seated, the rest of the screws need to be? I assume my faulty petcock could also contribute to that and I’ve already ordered one of those.
     
  2. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

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    Float height can be incorrect and you won't have fuel overflowing, so that's not an indication of anything other than the float needles are shutting off fuel flow before the bowls are overfilled.

    The petcock just controls fuel flow to the carbs. It does not meter fuel at all. So unless it's leaking, or not shutting fuel off, it's fine (for now anyway).

    Baseline for the fuel mixture screws is 2 1/2 turns out from a soft-seat.

    Backing up a bit, black and sooty plugs can be the result of several things. Some carb related, some not.
    Is the bikes intake and exhaust the same as it was when delivered from the factory?
    Does it use any oil?

    Also to be sure we're all clear moving forward we need to nail-down which XJ700 you have. There are two models, one air-cooled, and one water-cooled. They are very different beasts.
    Which do you have?
     
  3. Isaac G

    Isaac G New Member

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    I have the air cooled model. There is no oil use, checked daily. Previously had the issue of overflow fuel but fixed that to my knowledge. I read somewhere the float heights needs to be somewhere around 16mm? And the whole bike is bone stock except for an in-line fuse. Though I have removed the mufflers due to them not fitting properly and safety reasons of being heard and seen.
     
  4. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

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    That "loud pipes save lives" saying is a bunch of drivel. You're more likely to be punched by a pissed-off neighbor in your driveway at 5 am than be heard by a car on the road.

    Float heights are a baseline that are then used to set the fuel height.

    Setting the fuel levels

    When you fixed the fuel overflow did you replace the float needles?
     
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  5. tabaka45

    tabaka45 Well-Known Member

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    Once you get the float level correct or verified and assuming the carbs are thoroughly clean, the best way to set the pilot screws is with a colortune plug. Install the colortune plug in place of the #1 spark plug, get the engine warm and at idle. Close the pilot screw until you get white flashes which indicates lean, then open the screw until the white disappears. Repeat for all four cylinders. Install new plugs (easier to read new plugs than old one.) Ride about 20 miles or so and pull the plugs and see what they look like. You are looking for a light gray or brown color, not all white. If you do not have the light gray or brown open the pilot screw about the width of a dime. Repeat for all cylinders. Take another ride and repeat the process until you have perfect colored plugs. It took me a week or so and several adjustments to get the right color, but the engine runs much better and my miles per gal went up. I know that 2 1/2 turns open is the base adjustment a lot of folks use, but with using the colortune plug none or my pilot screws are more than 2 1/4 open and some at about 2 or slightly less. Every bike and carb will be different. If you don't have a colortune plug start at about 1 3/4 and make adjustments as needed.
     
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  6. MaximusXJ650

    MaximusXJ650 Member

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    How accurate is that statement? Can anyone comment?

    I ask because I'm having issues with power loss and I end up either running too rich or too lean. All my screws are at 2 and a half turns out and I was under the impression that they couldn't be different.

    So if I can have them at different settings I may be able to get the true balance I need
     
  7. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

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    Tabaka's statement is 100% accurate.
    Each cylinder is an engine of itself, and needs to be adjusted according to its individual needs. 2 1/2 turns out (fine screws, which your carbs have) is the baseline setting that will ensure the engine runs so you can then tune it.
     
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  8. MaximusXJ650

    MaximusXJ650 Member

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    Thank you! This is probably where I'm going wrong. I'm trying to keep the screws at 2.5 turns out on all of them!
     
  9. Door dude

    Door dude Active Member Premium Member

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    Now every time you adjust the screws will the carbs need to be re synced ?
     
  10. Nuch

    Nuch Well-Known Member

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    The accuracy of your fuel/air mixture screw position can be determined with a color tune plug as tabaka has suggested above. Yes there could be a bit of "back and forth" of adjusting the mixture screws and then re adjusting your vacuum sync. I'm sure others can chime in here with a more specific response on the actual "first this than that" procedure.
     
  11. XJ550H

    XJ550H Well-Known Member Premium Member

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    color tune, sync, color tune ,sync, color tune, sync until you are happy with results.
    if you adjust the mixture screws you should be syncing after do you need to? it all affects fuel mileage and performance .
    there are some who ride on the bench sync and never go past that ………...
     
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  12. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

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    The Law of Diminishing Returns applies. Generally once the idle mixture is set you can do the running synch and be done. Any further adjustment would be either because of an error during the first, or to smooth out the 4,500 RPM buzz. How perfect you need it to be to suit you is another variable.
     
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  13. Door dude

    Door dude Active Member Premium Member

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    Can you elaborate on the 4,500 rpm buzz k-moe ?
     
  14. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

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    The engine and frame reach they natural frequency and your nethers begin to tingle as a result. There are a ton of older threads about it.
    A running synch gets it calmed-down. In fact when I feel the buzz get worse I know it's time to synch (which has only happened twice in seven years).
     

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