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What is the hose on number 3 caburator holder ?

Discussion in 'XJ Technical Chat' started by thanh, Feb 20, 2013.

  1. thanh

    thanh New Member

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    I took off my xj 750 caburator for a long time so i forget where the hose is
    It's on number 3 i don't know what and where to connect the things .I sure that is not a fuel hose but i forget its. Once i've to noticed spark plug
    number 3 look pale white than the other i thought the hose is concerned.
     
  2. hogfiddles

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

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    That hose goes from the carb boot to the vacuum pipe on the petcock.

    Dave F
     
  3. thanh

    thanh New Member

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    Thank you guy . But i don't know its what for?
     
  4. quebecois59

    quebecois59 Well-Known Member

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    Petcock on an XJ is vacuum operated. It needs vacuum from the engine to open and let some fuel go down to the carbs bowls when the petcock is on ON or RES. Actually, this vacuum hose could be connected to any carb, but it is usually connected on carb #3.
     
  5. Foolber

    Foolber Member

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    Just a side note. I got a non vacuum petcock a couple weeks ago. Haven't got the bike done yet but don't see why it wouldn't work. Ill just plug he hole up like normal.
     
  6. quebecois59

    quebecois59 Well-Known Member

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    Non vacuum or manual petcock will work for sure. You just have to plug the vacuum bar of the carb #3, then make sure the petcock is ON when you ride and OFF when you don't, just in case your float needles aren't closing properly
     
  7. Foolber

    Foolber Member

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    yea iv left them on a couple time with my dirtbikes, woke up and could smell the gas in the kitchen lol. i have a shirt that says i love the smell of gasoline fumes in the morning :) so its fitting...lol
     
  8. mtnbikecrazy55

    mtnbikecrazy55 Active Member

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    Could be waking uP to your garage burning down, lo
     
  9. Foolber

    Foolber Member

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    one can only hope, this house is full of asbestos and mold, and i think the paint is so old it still has lead in it...
     
  10. RickCoMatic

    RickCoMatic Well-Known Member

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    Insurance against the Petc0k leaking and the Carb Floats failing and gas over-flowing or filling the Crankcase is cheap.

    A Briggs & Stratton Fuel Valve Shut-off can be bought at a Hardware Store.
    Mount it right next to the Petc0ck.
    "Paint" the Hose Clamps with Black Nail Polish.
    Paint the RED Valve with some Black Polish leaving the end of the Valve RED and the Shut-off virtually disappears.

    ::: Petc0ck With Aux Shut-off Valve On JX-750FM :::

    [​IMG]
     
  11. day7a1

    day7a1 Member

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    Your spark plug color is a direct result of your missing hose. You are sucking in air from #3 carb boot. You may also be getting detonation on that cylinder also. You need to fix that quick. You can go to the local auto parts store and get vacuum hose for $0.cheap, or you can get vacuum caps for $1.cheap. If the cap, just cap the connection on the boot. If you go with the hose, you run from the boot to the petcock (the smaller connection) and you will want to move from PRI to ON. This will also allow your reserve to work.

    There maybe problems with the petcock and carbs that would allow fuel to drain into your crankcase, so keep track of your gas mileage and oil level closely.
     
  12. thanh

    thanh New Member

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    Thank all you guy.My petcock is still draining the fuel nomatter ON or Res But motor is not running.Shall i plug on a #3 carb boot?
     
  13. quebecois59

    quebecois59 Well-Known Member

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    Your priority should be to order a rebuild kit for your petcock or a new manual petcock then go buy and install a cheap fuel valve as per Rick recommendation if you are not planning to fix your petcock soon.
     
  14. thanh

    thanh New Member

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    Thank you Quebecois59 .That's a great idea
     
  15. shangovi

    shangovi Member

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  16. RickCoMatic

    RickCoMatic Well-Known Member

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    The "Do-it-yourself" Instructional and Pictorial submitted by bigfitz52 is a comprehensive guide to rebuilding a stock petc0ck.

    The Kit, alone, costs nearly $40.00

    I believe it is a much wiser choice to invest in a Brand New Petc0ck.
    The old one lasted almost 30 years.
    You can expect a New one to do the same.

    There is, at least, an outside chance that after rebuilding your petc0ck with an aftermarket kit, ... it might still leak.
     
  17. rocs82650

    rocs82650 Well-Known Member

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    Rick,

    I'm curious. What makes you say that? I bought the kit from Len and rebuilt my petcock. I replaced all the seals and diaphrams. I check it periodically to insure it isn't leaking. I understand the chance that it might leak but outside of damaged parts, why would it leak? Knock, Knock, Knock...I seek light.

    Gary
     
  18. quebecois59

    quebecois59 Well-Known Member

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    Original o-ring of the diaphragm-activated fuel valve in the original petcock was tapered. O-ring in aftermarket rebuild kit is a regular, round section o-ring. It wouldn't seal as well as a tapered one, specially when the seat isn't perfectly machined, which was sometimes the case of the original petcocks.

    I'm pretty sure I have pictures that could help understanding that, I'll have to dig in my hard disc to find them...
     
  19. quebecois59

    quebecois59 Well-Known Member

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    "In addition to the shapes of the seals being substantially different, there are two more unseen differences. First, the stock seal is hollow while the replacement o-ring is not. And second, the stock seal is much more pliable than the stiffer o-ring used in the replacement part. These three differences contribute dramatically to the failure of the Maxim-X petcock rebuild kits... but they're not the only problem.
    Even if the seals were the same, the plunger found in the K&L kits is also a little shorter than the stock plunger. The difference isn't much, but it's enough to prevent the o-ring in the kit from making a solid seal against the valve seat... but that's still not the only problem.
    As it turns out, even if the seals were the same and the plungers were identical lengths, it's the machining of the body of the stock Taiyogiken petcock that's the fundamental, underlying problem"

    Found on maxim-x.com site, created by Harald Pfeiffer.


    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    I doN't know if all of these problems also occur with other petcock models, though, but this one is very well documented.
     
  20. bigfitz52

    bigfitz52 Well-Known Member Premium Member

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    The bigger difference is that the original o-ring had a hollow cross section making it much more "compressible."

    It IS true however, that not every petcock can be rebuilt; but it is generally more a case of deterioration of the valve seat area than a machining flaw.

    Provided the petcock has a good, viable seat and care is taken to ensure the valve piston sits and moves squarely, a rebuilt petcock can and will function just fine. But it takes "upping the ante" precision-wise, to be sure.
     

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