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idling issue/question

Discussion in 'XJ Technical Chat' started by porter7o, Oct 5, 2014.

  1. porter7o

    porter7o Member

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    Carbs rebuilt
    Valve clearances in spec
    Floats in spec
    Bench synced

    Xj750 maxim

    1) I reassembled, and it started no problem, but idled extremely high between 5-6k rpm.

    I had the idler screws turned out 2.5 turns from bottom, is that right?

    Drove around the block, it was slow to accel, but worked better than before the rebuild.

    I tried to turn the idler master/butterfly adjustment screw but could not bring down the high rpms.

    2) I pulled the carbs back off and re bench synced, better this time. I also bottomed out all 4 idler enrichment screws per the yamaha book saying that it was set at manufacturing, which I thought was wierd.

    Now it wont start up. I could barely see a sliver of light through the butterflies.

    Do i need the 2.5 turns of idler screws?

    I cant vacuum sync or colortune without the correct idle speed.

    Thoughts?

    Thanks!
     
    Last edited: Oct 5, 2014
  2. Stumplifter

    Stumplifter Well-Known Member

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    The 'idler screws turned out 2.5 turns from bottom' sounds right for a starting point.
    So there is no misunderstanding can you post a picture of where these screws are on your rack?

    The Main Idle Adjustment Screw (at least on a Hitachis set of carbs) is in between carbs 2 & 3, it is horizontal in position and on the air box side of the rack. It can be turned by hand, if you have skinny fingers, turning counter clockwise will slow down the RPM's.

    So, with what I think I understand from your description, you should:
    1. back out you mixture screws to 2.5 turns out
    2. use the Main Idle Mixture Screw to bring down the RPM's
    3. continue with vac sync and colortune
     
  3. porter7o

    porter7o Member

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    The idler mixture screws that i am talking about just got new springs/washers/seals, the really tiny ones that you need tiny leprechaun fingers to replace.

    I will turn those out 2.5 turns and adjust the main idle screw between #2 and #3 per your suggestion if needed.

    Once it is warm i have fresh oil and a new filter as well as the rest on the sync to do.

    Thanks, I will keep you updated in a couple hours.
     
  4. porter7o

    porter7o Member

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    So I did this, it started without even putting the 'choke' on and idled at 6-7k and no help from the middle adjustment screw.

    Maybe I don't know how to bench sync...

    Also, I sprayed around the boots with wd40 to check for leaks, no jump in rev speed. I did this quickly so as not to overheat it.
     
  5. quebecois59

    quebecois59 Well-Known Member

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    Here is the Main Idle Adjustment Screw (or knob):

    XJ750K-vis ralenti-petite.jpg
     
  6. k-moe

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

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    The screws tht you are calling the "idler adjustment screws" are the idle mixture adjustment screws. As stumplifter mentioned a good starting point is 2.5 turns out from fully seated. These screws adjust the fuel mixture at idle. It sounds like you closed them entirely, blocking off the idle fuel circuit, hence your current no-start situation.

    Your engine idle speed is adjusted with the control knob pictured in quebecois59's post.

    Bench synch instructions. http://www.xjbikes.com/forums/index.php?threads/how-to-bench-synch-your-carbs.6366/

    Make sure that you have the cnotrol cables hooked up correctly. You would not be the first person to connect the throttle cable to the choke arm.
     
  7. porter7o

    porter7o Member

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    Thanks for the advice, turns out that the PO needed a buttload of throttle so I only had to adjust it down at the handle about 20 turns.

    Major syncing and tuning to take place tomorrow.
     
  8. fiveofakind

    fiveofakind Well-Known Member

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    Carbs rebuilt.......when you performed this exercise did you "break the rack" apart & replace the throttle shaft seals & fuel rail o-rings ?

    What do you mean by carbs rebuilt ? Please elaborate on this .....
     
  9. porter7o

    porter7o Member

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    I did all i could to the carbs without braking the rack, so it is considered a partial rebuild, right?

    The problem went away with the adjustment at the throttle cable. Now it idles barely above 1000 rpm once warmed.
     
    ecologito likes this.
  10. quebecois59

    quebecois59 Well-Known Member

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    20 TURNS !!! How could that be?
     
  11. k-moe

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

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    How could that be?
    That could be hyperbole!
     
  12. porter7o

    porter7o Member

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    Seriously, he probablyv thought it solve the bikes previous issues. Even bandaids need replacing after a while.
     
  13. porter7o

    porter7o Member

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    20 half turns would be more realistic ;)
     

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