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  1. #1
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    OK, so I found a 1978 CX500 Deluxe tank that I'm going to have overhauled and sealed and I removed the petcock only to discover it has one hose going out while my 1981 CX500C has 3 hoses attached to it. Are they interchangeable on the size of the attaching nut or am I going to have to do some sheet metal welding to make this work? Or am I missing something obvious?- TT
    1981 CX500C

  2. #2
    Super Moderator Blindstitch's Avatar
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    The petcocks are different threads. The older is 16mm and the later is 18mm.



    It's probably easier to buy the right one from honda and plugging the vacuum hose that runs from the right carb to the petcock.
    1968 Honda CT90 Trail-Yellow Monster project
    1979 Honda CX500 Supertanker 72,000 miles.
    1983 Honda Gl650I Falcon-killer 70,000 miles.
    Pictures of my bikes Past and Present.



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  3. #3
    Senior Member Allan's Avatar
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    The extra hoses on the 81 are because it has a vacuum petcock,,you can use the petcock that is on the 78 tank, you just have to block the vacuum port off on the right hand carb.
    ...and as the elders of our time choose to remain blind, let us rejoice and let us sing and dance and ring in the new. Hail Atlantis!

    1975 XL-100, 1978 CX500, 1980 CX500, 1981 CX500

  4. #4
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    Quote Originally Posted by Blindstitch View Post
    The petcocks are different threads. The older is 16mm and the later is 18mm.



    It's probably easier to buy the right one from honda and plugging the vacuum hose that runs from the right carb to the petcock.


    I've seen them on ebay cheap. I have a couple I was going to sell but I see they're practically given away. So I decided to keep a dented up D tank I have and try to save it. Best of luck. I've had good success with POR-15 products, I actually used their paint to seal an old CT90 tank I have. It's been almost a month now with gas in it with no ill effects.
    2008 CRF100

    1984 Kawasaki KLT250 (aka 3-wheeler of death)

    1999 Honda 250 Fourtrax

    1973 CB350F

    1975 CB550K

    1983 GL650I

    1973 CB750K

  5. #5
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    Quote Originally Posted by fmctm1sw View Post
    I've seen them on ebay cheap. I have a couple I was going to sell but I see they're practically given away. So I decided to keep a dented up D tank I have and try to save it. Best of luck. I've had good success with POR-15 products, I actually used their paint to seal an old CT90 tank I have. It's been almost a month now with gas in it with no ill effects.




    The prices on petcocks are all over the damn'd place! Like you said- as low as $10 plus shipping, and as high as $55 plus shipping. So after checking out Ebay I was a bit bewildered shall we say? Thank You- every one of you- for telling me the olders were 16mm and the newers at 18mm.
    1981 CX500C

  6. #6
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    Quote Originally Posted by Trailltrader View Post
    The prices on petcocks are all over the damn'd place! Like you said- as low as $10 plus shipping, and as high as $55 plus shipping. So after checking out Ebay I was a bit bewildered shall we say? Thank You- every one of you- for telling me the olders were 16mm and the newers at 18mm.




    I'll look at the one I have on a C tank. No idea of the condition though, it was on a rolling frame I picked up with my CX. If it works, you can have it.
    2008 CRF100

    1984 Kawasaki KLT250 (aka 3-wheeler of death)

    1999 Honda 250 Fourtrax

    1973 CB350F

    1975 CB550K

    1983 GL650I

    1973 CB750K

  7. #7
    Senior Member Allan's Avatar
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    Quote Originally Posted by fmctm1sw View Post
    I'll look at the one I have on a C tank. No idea of the condition though, it was on a rolling frame I picked up with my CX. If it works, you can have it.


    Unless I'm reading his first post wrong,,I believe he already has a petcock for the 78 tank.



    Quote Originally Posted by Trailltrader View Post
    OK, so I found a 1978 CX500 Deluxe tank that I'm going to have overhauled and sealed and I removed the petcock only to discover it has one hose going out while my 1981 CX500C has 3 hoses attached to it.




    He can use that petcock, but he will have to plug the port on the right hand carb if he wants to fit the 78 tank on his 81.
    ...and as the elders of our time choose to remain blind, let us rejoice and let us sing and dance and ring in the new. Hail Atlantis!

    1975 XL-100, 1978 CX500, 1980 CX500, 1981 CX500

  8. #8
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    OK- heres the deal: The 81 currently has its own orginal "C" tank and its petcock works fine. The doner bike is a 78 and its going to have its tank overhauled to kick the bunkerage up from 2.71 to 4.5 gallons using the "D" tank. The "D" tanks petcock is pretty trashed from corrosion but it did pass the "blow air through it" test. Its internal gasket is trashed however- I could rebuild it? But I'm thinking my times more important than that.



    I love my Honda. I hate the bastard who thought putting a 100 mile range on a motorcycle was a good idea. I love the idea of strangling this person and force she/him to ride across the United States with only a 100 mile range tank.
    1981 CX500C

  9. #9
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    The `78 Petcock is way smaller, I just bought a `78 tank and put it on my `81 and the bike leans out at high speed. The size of the petcock is the only variable.

    Now I have to come up with an adapter for the `81 pet ccock so I can mount it on the older tank.



    Probably have to buy a Pingel.





    This is the pic of the petcock comparo 1981 vs 1978

    http://smg.beta.photobucket.com/user...s1978.jpg.html
    `81 CX500 D, `00 KX500, `95 Trident, `77 GS750

    Three dogs and a cat

    Drive a Ranger, Never voted for President Ebola.

  10. #10
    Senior Member Rusty Bikes's Avatar
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    I must have a 78 tank on my 81. My petcock does have the vac port which I have by passed. It looks more like the smaller of the two in the picture. It sure does not have a bowl. The valve on my new 78 looks about like the smaller of the 2 as well.

    But I have not done any measuring. I doubt the petcock can lean out your bike at highway speed. There must be something else going on to restrict flow.



    I may be comparing apples to oranges since I am looking at Custom models.
    Rusty Bikes



    1981 CX500C "Soup Line Sprinter" Post apocalyptic mayhem machine and daily driver

    1979 CX500C Dirty whore doaner bike with messed up title issues (Dismantled)

    1980 Honda Express NC50 no-ped

    1988 Yamaha Riva 125

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