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acceleration flat spot

Discussion in 'XJ Technical Chat' started by joejr2, Aug 20, 2021.

  1. joejr2

    joejr2 Active Member Premium Member

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    After getting the starter to fire, swapping out a bad cutoff relay, the bike starts up without much enrichment.
    The xj650 seca rj idles smoothly at about 1k-1.5k. There is a flat spot at the bottom end at about 2k when the
    throttle is goosed this is accompanied by a slight pop. would that indicate rich or lean mix ?
     
  2. tabaka45

    tabaka45 Well-Known Member

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    Are you running pods, or OEM air filter?
     
  3. joejr2

    joejr2 Active Member Premium Member

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    stock airbox with good rubber manifolds (tested with starting fluid no leaks) KN paper filter. Starts up with just a little enrichment
    off enrichment about 1 minute. idles smoothly at about 1-1.5k. If I goose the throttle it hesitates at about 2k with a slight pop
    intermittantly. Then drops back to idle slower than my other xj650 but doesn't hang up. Does that show rich or lean on the mixture ?
    I'm in Santa Fe at about 7300' elevation.
     
  4. tabaka45

    tabaka45 Well-Known Member

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    I'm at sea level so I don't have any experience with high altitude adjustments. However, I think that I have read that high altitude requires some upsizing in the jets. Some of the gurus will have to give you more information on that. My guess is that it is a little lean. It's a real plus that the bike is stock, although I am assuming that the KN paper filter is not a "high flow" filter, which would certainly result in a lean mixture. If you have not cleaned the carbs that might be in order and at the same time verify the correct jetting for altitude. Chacal or a service manual should be able to give you that information. I assume that you have synced the carbs.
     
  5. joejr2

    joejr2 Active Member Premium Member

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    The carbs are recently rebuilt with stock sized jets. You think the flat spot and intermittant "pop" on acceleration indicates lean ? I've
    got the mix set at 2.5 turns out. I'll fiddle with the mix a little before I synch I have a couple of vacuum gauges
     
  6. tabaka45

    tabaka45 Well-Known Member

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    I am surprised someone else hasn't chimed in about the jets needed at altitude. If "stock size jets" means those that came with it for sea level use, that could be the problem. By the way, what do your spark plugs look like? They should have a nice light gray or brown color on the porcelain. If they are lean the porcelain will be white.
     
  7. tabaka45

    tabaka45 Well-Known Member

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    I just Googled "carburetor adjustments for altitude" and found a lot of information that might interest you. Because the air is less dense, or thinner, smaller jets are used to allow less fuel and compensate for the thinner air. So my first comment about upsizing the jets was wrong. They would need to be downsized. One source suggested 1 size down for every 2000 feet of altitude. I suggest you google "carburetor adjustments for altitude" and see what you find. Hopefully, someone on this forum with experience at altitude will jump in.
     
  8. k-moe

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

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    The jets shouldn't need to be changed at all for that altitude.
    There were (so for as I'm aware) no jetting changes made by Yamaha for bikes sold in high-altitude locations.
    Since you have another 650 that is running well, you could check the jets in that one to see if it's jetted differently.

    Double check that you have the air jets in the correct locations, as reversing them can cause the condition you are having.
    [​IMG]
     
  9. joejr2

    joejr2 Active Member Premium Member

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    Got all those jets in the correct places. Any how, I'm leaning to rich condition as to the flat spot and minor popping/ I couldn't find any air leaks
    and at 7300k alt. and 2.5 turns out on the mix screws, would be a liitle rich. I wanted to know what forum folks thought. I'm gonna start by leaning the
    mix down a hair at a time to see if that improves the flat spot or makes it worse. Then synch them
     
  10. Rooster53

    Rooster53 Well-Known Member Premium Member

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    I think I remember you said you pieced this one together from several sources. This is not a likely fix but I do know I have tested a couple of TCI's from the 650 Seca in my 750 Seca and experienced a flat spot at that low RPM. It was most noticeable coming out of a corner when the throttle was opened and was just a short duration miss. If you happen to have 750 pickup coils or incorrect TCI there may be a combination where this works somewhat backwards to create what I was experiencing.
     
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  11. joejr2

    joejr2 Active Member Premium Member

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    TIC is good I tested it in another running 650 maxim
     

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