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82 750 running lean on one hole

Discussion in 'XJ Technical Chat' started by Tyson Krall, Jun 14, 2019.

  1. Tyson Krall

    Tyson Krall New Member

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    Hey guys, I have a 1982 xj750, got it running good on all 4 holes at an idle, but when I go down the road and crack it to WOT, it sputters and falls flat. We did a full throttle kill test when it was sputtering and found one hole was very lean and the other 3 were fine. I took carb off again, all was cleen, (just came back from Yamaha getting synced and a that good stuff). Replaced vacuum line from carb 3 to fuel valve and tried it on carb 4 for the hell of it. No difference. All the boots look good and we did the ether test to make sure and everything checked out. What’s your suggestions to try next? The Yamaha dealers I call are very puzzled by this, as the jets and everything look good.
     

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  2. mlew

    mlew Well-Known Member

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    3 look rich the other looks almost normal. Have you checked the fuel levels by wet setting the floats and the carb slide diaphrams for pin holes?
     
  3. Tyson Krall

    Tyson Krall New Member

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    I had all that done at Yamaha, It’s definitely leaning out cuz it falls flat on its face but can’t fogure out why
     
  4. k-moe

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

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    They didn't do shit, or at least didn't do it correctly. If they had you'd have no problems.
    YOU are your own best mechanic.

    Falling flat is a sign of richenss, not leanness.
    While dark plugs can be a sign of leanness, more often it's a sign of being overly rich.

    One quick thing to check is the position of the air jets in the top hat. The service manuals (ALL of them) have their positions mislabled.
    This is the corrected installation guide:
    [​IMG]

    Reversing those jets will cause an improper fuel mixture above idle. Remove the top hats (carbs can stay on the bike for this, just pull the tank), check the jet positions, and if they need to be swapped you will also need to reset the idle mixture screws. 2 1/2 turns out from a soft-seat is baseline, and most bikes end up between 2 3/4 to 3 turns out to get the idle mixture right.

    Then you have some homework to do so you can stop having to pay other people to not fix your machine.

    Start here: Read This First
     
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  5. k-moe

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

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    One more thing. Please keep all commments and questions about fixing your bike in this thread. Spreading info around multiple threads makes it really difficult to keep track of what has been done, and that makes it harder to help you.
     
  6. Tyson Krall

    Tyson Krall New Member

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    Thank you! I will give this a look over and check. A simple air jet mix up could be the issue. All other 3 cylinders look good so I’m banking on that being the Cause. I know it’s not a proper way but when I drained all 4 carbs the bowls seemed to have the same amount of fuel in them
     
  7. Tyson Krall

    Tyson Krall New Member

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    Here is the air jets off that carb. Looks right to me
     

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  8. k-moe

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

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    Again, the carb that you think is lean isn't lean enough to cause any problems (not in the short-term anyway).
    Check the other three, because all of them are where the problem lies.
     
  9. Tyson Krall

    Tyson Krall New Member

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    Okay, sorry if I misunderstand a few things as I’m very new to this bike and carbs in general. You think that maybe that sputter and falling flat (not gaining speed at all while sputtering) could possibly be from being to rich? Which could possibly be from improper air jet location on the other holes, or wore out fuel jets?
     
  10. raskal

    raskal Active Member

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    Yeah. He said that a couple of posts above

     
  11. Tyson Krall

    Tyson Krall New Member

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    Wow I completely missed that part. Guess I just looked at the pics and went from there. Thanks for the Clarification
     

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