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High RPM's after rebuild

Discussion in 'XJ Technical Chat' started by Ross1bd, Aug 12, 2015.

  1. Ross1bd

    Ross1bd Member

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    Just rebuilt Mikuni's on the new to me '83 900. Valves checked by the most recent PO. With the throttle cable disengaged and the idle screw backed off fully it starts w/o enrichment and runs at 4k.
    4 into 1 exhaust, mains are still 102.5's. I haven't checked the plugs, only have 4 or 5 minutes running time (makes me wish for an oil cooler on the 750). Disassembled, new throttle seals, new springs, O-rings and washers on the pilot needles (which are backed out 3 turns). Zestfully clean, clunks like a lead balloon.
    Cylinders 2 & 3 are maybe 1/2 mm. low (3.5 in the range of 5 +/- 1 mm) the outer pair are 4.5-5 mm's.
    If I press down on the throttle linkage it'll slow by a couple hundred rpm. Sprayed ether on the boots, no change.

    Not sure what to try other than turning the pilot screws in to 2.5 turns, or even 2. I read many of the carb rebuild threads and many of the trouble starting ones. Only saw the pilot screws mentioned as needing to be turned out?
     
  2. jayrodoh

    jayrodoh YimYam Premium Member

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    When the carbs were off, did you verify that the throttle plates would close all the way after reassembly?
     
  3. Rooster53

    Rooster53 Well-Known Member Premium Member

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    ^^^^ and did the proper bench sync to ensure all the throttle plates are set equally for a good starting baseline before doing a running sync. It's possible for one carb throttle plate to bottom while the others are still open, at that point backing off the idle screw will not accomplish anything.

    Here is a link for Hitachi carbs, but the concept is the same. The paperclip is a bit large in this method, so when completed back off the idle adjustment knob and make sure all the throttle plates will completely close.

    http://www.xjbikes.com/forums/threads/how-to-bench-synch-your-carbs.6366/
     
  4. XJ550H

    XJ550H Well-Known Member Premium Member

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    Last edited: Aug 12, 2015
  5. k-moe

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

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    Has nothing to do with your current problem, but don't count on that unless he provided you with a table of the clearnaces and what shims are installed. Een then you should confirm clearances yourself.

    Now to your problem:

    The pther guys have most things covered, but...

    When you cleaned the carbs you were sure to put the air jets back where they came from?
    The pilot screws are set to 2.5 turns from lightly seated?
    The slides move freely?
     
  6. hogfiddles

    hogfiddles XJ-Wizard, Host-Central NY Carb Clinic Moderator Premium Member

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    Shims were done at one of my carb clinics a couple years back, we did several bikes that day, and unfortunately the one thing that wasnt recorded was what page had been for which bike. Spat the end of the day I couldn't ID which page was for which bike. I wasn't worried, because I was figuring that AJ ce two of them were mine , and a third was a few miles down the road, I'd just end up re doing them in a few years anyway. This bike ended up being sold, another is still asleep on my lift, and the third on has gone from a few mikes down the road to a few mile UP the road..... That one will be due for a clearance check fairly soon. Ummm, ARE YOU READING THIS, ELLIOTT?!?)---- lol
    Long and short, valve clearances are good... I only rode maybe 100 mi after valve shim job. I just got busy with things and figured I'd get back to the bike after awhile.... Well--Ross1bd got to it before I did..... Lol

    The 4/1 didn't seem to bother the performance of the bike.... It had just as much giddy-up as my show 900rk. Only difference I noticed was the exhaust was louder, and to my ear sounded weird because of it being only on one side of the bike rather than mufflers on both sides.

    To the point, it sounds like a butterfly is still holding things open..... Could also be:
    -Cable hanging up, or not seated quite right
    -air jets inadvertently mixed up
     
    Last edited: Aug 13, 2015
  7. k-moe

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

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    The one time when you really can trust the PO...
     

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