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XJ700N Spark Plugs

Discussion in 'XJ Technical Chat' started by Dan Gardner, Jun 18, 2021.

  1. Polock

    Polock Well-Known Member

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    get off the highway and stalls at red light, is a classic valve adjustment problem
     
  2. Mechanic1978

    Mechanic1978 Active Member

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    @Toyobaru866, he's right in saying it controls the "mixture"....the carb will either pull air or fuel in the pilot circuit automatically based on that cylinders vacume level,
    but not both. So if the external "Pilot"adjuster is mounted "High" up on the carbe base, then it will adjust Air, for the mixture and carb will most likely be pulling fuel automatically if circuit is not clogged.

    If the Pilot adjusting screw is mounted "Low" on the carb base, then it will adjust "fuel", For the mixture and pull air automatically.

    thats typical for most bike carbs in general, however some models of carb have an external "Brass" tube running fuel from bowl to upper part of the carb, also some have accellerator pumps. so not all are alike but general function is common among all...
     
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  3. hogfiddles

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

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    It can be a symptom..... But it can also be a symptom of too much fuel swamping everything.

    Getting the carburetors cleaned out, rebuilt, and dialed back in, Removes them from the problem list
     
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  4. Toyobaru866

    Toyobaru866 Active Member

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    Yes I know he's right :) I don't doubt hogfiddles one minute! I didn't express myself good in my last post. But thanks for explaining in detail, always willing to learn!
     
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  5. Toyobaru866

    Toyobaru866 Active Member

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    Another proof of not expressing myself very good... Of course I meant 'for a second'....
     
  6. Dan Gardner

    Dan Gardner Well-Known Member Premium Member

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    I did a quick sample of #1 and #4 float heights on the XJ650J (which turned over to 30,000 miles today, just a fun fact).

    Here's what I found:

    #1 is AT LEAST 10mm below carb body:
    carb 1.jpg

    #4 is better, but still at least 5mm below:
    carb 4.jpg

    So I think the plan will be to pull the rack, verify the size and position of all fuel and air jets, and set float heights. Not sure when I will do it, would prefer to wait until I have XJ700N rideable again.
     
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  7. Mechanic1978

    Mechanic1978 Active Member

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    @Dan Gardner, when you pull the carbs take the floats off, and get a clear container full of water large enough not to allow the floats to touch anything.

    stick each float in the water and see how far it sinks, place them in with pivot bracket down gently so as not to allow them to bob around.. the floats over alot of heat cycles can absorbe fuel like a sponge would. compare the amount of float above the water line to each other.. if you can get all 4 in one container at same time, that will be easier.

    Also get a small kitchen scale to weigh them, these test will help you to determine if you need to replace one or two.
    there is no set amounts here, but more a comparison between the 4.. this will also allow you to set float heights.

    bet the manuals dont help anyone like that...lol
     
  8. hogfiddles

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

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    FWIW, I have NEVER seen an xj float absorb fuel
     
  9. Mechanic1978

    Mechanic1978 Active Member

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    @hogfiddles, your way more familiar with the XJ's than I... but it can happen, I've seen it before. but not necessarly on any XJ's. so I will take your word for it.

    float them next time you do a carb set, i would bet money you will see a difference on depths they sink across all carbs in the rack.
     
  10. hogfiddles

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

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    I have see it on other things, for sure.... never an xj though
     
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  11. Mechanic1978

    Mechanic1978 Active Member

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    I wish XJ's had brass floats... the ones you have come across, have you ever had any xj carbs with brass?
     
  12. Franz

    Franz Well-Known Member

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    So if floats can absorb fuel would that mean the needle valve would not shut resulting in a flooded carburettor?
     
  13. Mechanic1978

    Mechanic1978 Active Member

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    no it would result in a bowl never getting to set bowl height. heaver float will be lower in fuel level.

    Edit: correction, yes it would. dont know what I was thinking there. float would hang low in bowl, not letting the needle close as soon as it needed to. thanks @hogfiddles, I read you last post there and rethunk it.
     
    Last edited: Jul 15, 2021
  14. Franz

    Franz Well-Known Member

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    Yes that makes sense now.
     
  15. hogfiddles

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

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    No
     
  16. hogfiddles

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

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    Very likely, as the float would likely not rise high enough to close the valve...... unless you’ve wet set the “heavier” float, in which case you compensated for it by setting the fuel level to the correct height
     
  17. Franz

    Franz Well-Known Member

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    That is what I originally thought. If the float is sinking in the fuel the needle valve would not close because the tab on the float is lower because the float is lower. So if XJ floats did this even with the fuel not on prime at the tank valve the carburettors would flood even with good needle valves and seats.
     
  18. hogfiddles

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

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    If the valve is not closing all the way, the carbs will fill too full, possibly leading to flooding if it doesn’t finally close.

    if the petcock is not on PRI, NO fuel should flow unless the bike is running. If there is fuel still flowing when the petcock is set to run or reserve and the bike is not running, then there is a problem with the petcock. In theory, the float valves should close tightly enough so That The fuel will be stopped even with a petcock set on prime. This is also why it is not uncommon for people to put a shut off valve in the fuel line As an added precaution
     
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  19. Franz

    Franz Well-Known Member

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    I have not had any problems yet with fuel getting into the crankcase and l might fit a shut off valve, good idea.
     
  20. Mechanic1978

    Mechanic1978 Active Member

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    the petcock pump is not actually providing any PSI, its simple to assist in a faster flow under heavy throttle.
     

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