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Carburetor leaks only at high RPM's

Discussion in 'XJ Technical Chat' started by MrSeca, Nov 7, 2020.

  1. MrSeca

    MrSeca Active Member

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    I've been having this very unusual problem on my xj900 since I bought it 8 months ago: gas drips out of the 4th carburetor overflow tube ONLY when riding on the freeway above 5000rpms. If I'm just cruising around neighborhoods, it will not leak. It will not leak if I leave the petcock on PRI . It will not leak when it just sits there at idle. It will ONLY do it when I'm taking the bike at least 70mph for a duration of 2 minutes or more. I'll just hop on the freeway and after a minute or two I'll look down and there it starts, drip drip drip, but as soon as I get off the freeway and I come to stop light I'll look down and the dripping has seized.

    I bought new rebuild kits. I double, triple, quardruple checked for leaks. I spent hours measuring the float heights using the clear tube method and they are perfect. I ultrasonic AND chem dipped the carb bodies. Changed O-rings, throttle shaft seals. The carburetors were synched correctly using a vaccum gauge and the YICS tool. Why would an abundance of fuel fill the float chambers only when the bike is at faster speeds for an extended period of time and not when I drive at slower speeds in neighborhoods or around town?
     
  2. Franz

    Franz Well-Known Member

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    Needle height on CV slides?
     
  3. MrSeca

    MrSeca Active Member

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    Hmmmm. Never thought of that.
     
  4. MrSeca

    MrSeca Active Member

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    When I broke the rack and took out all the pistons, the chances that I put the same exact piston back into the carb it was in before is obviously a 25% chance but I still have the stock needles on the pistons and they are set with a single setting. But even if the needles were incorrectly placed how would that bring more fuel into the carb bowl to make it overflow?
     
    Last edited: Nov 7, 2020
  5. Franz

    Franz Well-Known Member

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    It's strange how it is only happening at higher engine rpm. I just wondered about the needle height since you haven't mentioned them. On reflection l think they height only controls the richness or leanness. You have done all the work and checked everything. With the carburettor only flooding at higher rpm it cannot be a stuck float etc.
     
  6. FJ111200

    FJ111200 Active Member

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    http://iwt.com.au/mikunicarb.htm
    Maybe of some help to you or someone else. It's not just for 2 strokes so don't be put off.
    I would leave it and see if it clears up. What's it like on an extended high speed run?
     
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  7. k-moe

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

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    My first thought is a float that isn't moving entirely freely, and gets stuck as engine vibration increases.
     
  8. Franz

    Franz Well-Known Member

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    Tapping the float bowl with a screwdriver handle might free it prior to a high speed run.
     
  9. Simmy

    Simmy Well-Known Member

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    I’m having trouble understanding why the motor is not sucking all this fuel into the cylinders at higher RPMs?
    I suspect the leak is external.
     
  10. Simmy

    Simmy Well-Known Member

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    I just read your post again.
    could it be that wind is blowing across the end of this overflow tube and drawing gas out?
    Try shielding the end of the overflow tube from the wind.
     
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  11. MrSeca

    MrSeca Active Member

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    Some how gas is getting into the overflow pipe
    OMG! That HAS to be it! Since I have owned the bike I never had a tube attached to the overflow nipple. It's just a bare nipple. The wind must be flowing right over it creating the Venturi effect and drawing gas out of the bowl. Makes perfect sense. Holy !@#$. This also has to be the reason my MPG has been affected as well. I will try this as soon as I can and will get back to you. Thank you so much!
     
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  12. Simmy

    Simmy Well-Known Member

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    I strapped my mercury carb synch stick to my bike and rode to the next town to help a guy tune his bike.
    I had to pull over on the expressway when I saw the mercury dripping out of the tubes.

    I still think your float level needs to be too high for this to happen. What happens when you apply vacuum to this tube at rest?
     
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  13. hogfiddles

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

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    So NOW you finally tell us your carbs aren’t completely done.........

    At least the conversation didn’t go like this:

    “I don’t know what wrong with my bike.......I did a through clean and rebuild, but it just won’t start now!”

    “ok, did you do “X, Y, Z.......?”

    “Yes, it just won’t even fire up at all!!”

    “Umm... got fresh gas in the tank?”

    “Yes, but the fuel line just hangs beside the carb rack because whenever I try to start it, gas runs out........”
     
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  14. MrSeca

    MrSeca Active Member

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    It's official. Venturi effect was in full swing at high speeds. What a kick in the pants. To test I put a piece of clear plastic tubing on the overflow nipple so I can monitor if fuel was still leaking out, hopped on the freeway and went about ten miles at about 75-80mpg. No fuel leakage the whole time. Not even a drop. By the way, I re-checked the floats and they were actually just a tad low so I don't thing it matters on the float height necessarily. There you have it.

    Next test is to ride a hundred miles so I can see how much my MPG has improved. I've been averaging 35-38 and I've spoken with people who have gotten as high as 55MPG. This should be a litle interesting.
     
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  15. hogfiddles

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

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    I stay about 48 mpg
     
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