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Left cylinder not firing / throttle issues. Help

Discussion in 'XJ Technical Chat' started by Jacob Paradise, Aug 22, 2020.

  1. Jacob Paradise

    Jacob Paradise New Member

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    I have a Yamahaxj650 H, 1981 with a left cylinder that won't fire and I can't give it throttle without it stalling. I replaced the plugs and boots, it gets spark and the cylinder has yet to fire at all. I'm not sure if the throttle issues are connected, but in combination to stalling, sometimes the rpm spikes to 4,000 on a start. The carbs are clean and rebuilt, everything looked good, fuel in the bowls, new air filter but it's still just not happy about something.
     
  2. k-moe

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

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  3. Jacob Paradise

    Jacob Paradise New Member

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    Yeah I got the carbs separated and all apart, the more I read about it, the more i suspect it could be a vaccum issue
     
  4. k-moe

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

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    Did you replace the throttle shaft seals?
    The fuel rail o-rings?
    The air jets in the correct locations (not where the service manual says to put them)?
    Bench synch done?
    Float levels wet-set?
    Did you remove the idle mixture screws, clean the ports, and replace the o-rings with new?
     
  5. Polock

    Polock Well-Known Member

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    try a running sync and see what happens
     
  6. Jacob Paradise

    Jacob Paradise New Member

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    I'm not even sure where the idle screws are on the carbs, I'm pretty sure the caps on top near the choke valves are just tamper seals for them. Would the jets and orings really prevent the entire piston from firing? It gets good compression and spark, and it fires when it's on starting fluid so I was just running off the assumption that the carbs just aren't getting enough gas in general and the vaccine line on the petcock or the petcock itself might be junk. The floats seem to be functional.
     
  7. k-moe

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

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    The anti-tamper plugs being in place is a clue that the carbs have never been properly serviced at all.
    If the idle passages are blocked there won't be enough (or any) fuel to ignite.
    If the rubber seals are perished there will be vacuum leaks (the throttle shaft seals are the primary culprit, but all rubber seals should be changed).

    Go back and read the links I posted earlier, pull the carbs, and revisit your work.

    You have to tear the carbs down entirely.There are no shortcuts to getting a old bike running well after it's sat unused for a long time.
     
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  8. Minimutly

    Minimutly Well-Known Member

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    Carbs rebuilt and clean - not by you obviously?
    Start the journey to knowing what it takes to rebuild these simple but horribly small orificed devices, then learn how to bench set and wet set the butterflies and the floats. If your manifolds are sealed it will start and run well enough to do a running synch. (Both my first attempts were ridable without running sync, you just need to be fussy).
     
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