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  1. #1
    CB750 Member leo750k's Avatar
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    Post 1980 cb750k, with 1981 motor, need carb O-ring & jet sizes

    Getting ready to ditch the pods, and go back to the OEM airbox. I will be going thru the carbs again, want to double check a few things.

    When I rebuilt them the first time, the O-rings for the fuel cut offs were the wrong size, diameter was ok but the cross sectional thickness was thinner then the c-bore in the carb body. so I reinstalled the old ones. (ya I know, it was a blond moment), also in my Honda factory manual it states "flat side twords carb body",?? is this a special style o-ring?

    And the dia. on the o-rings for the fuel and air tubes seemed a little small too. they seemed to fit tight enough into the carb body (I think). I also had trouble with one of the new needles & seats, for the CV piston, it was sticking 25% open, (needle was to big for the seat) so I reinstalled one of the old needles. needless to say I'm not trusting any part that came with that rebuild kit.

    What I am looking for is someone that might know the specific sizes for all the o-rings in the carbs, the factory jet sizes, and a trusted source for good quality carb kits for my 1980 CB750K, with I think 1981 motor, with (fuel line diaphragm valve) this component is only shown in the 1981 addendum for the cb750k, cb750f, and cb750c, and I did know the motor was not the original, but old owner never said it was a different year.
    thanks for everybody's help, info, insight, and suggestions,
    Sincerely,
    Leonard

  2. #2
    CB750 Guru amc49's Avatar
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    Motor could be the original , the carb bank may have been changed. Often how that fuel shut-off valve shows up.

    Here for carb rebuild manual. The O-ring sizes are detailed too.

    http://www.cb750c.com/publicdocs/Sea...anual_revG.pdf

    Thinking Randakk still sells the quality carb kits. There is NO special flatsided o-ring, have no clue why a manual would say that. Point out the page if a Honda OEM manual and I will look it up. There is no rebuild diaphragm for the fuel valve, most people drop them and learn to always cut the petcock off.

    The OEM primary jet is 68 and the secondary is 102.
    Last edited by amc49; 10-03-2019 at 03:54 PM.

  3. #3
    CB750 Member leo750k's Avatar
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    I have the original motor also, the previous owner replaced the whole motor because he said the original motor was making a knocking noise, but I just assumed it was replaced with the same year and model motor. I was in a hurry when I was typing the last post, its not the fuel cutoff, its the air cutoff (my mistake). The statement is on page 4-7 at the bottom in a box, and reads "when installing the air cutoff o-ring make sure the flat surface is toward the body"
    thanks for the help, will check out Randakk

  4. #4
    CB750 Guru amc49's Avatar
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    If talking about the aircuts under the cover on left side of each carb then yes, some were cut with flat sides on BOTH sides that I've seen. The small one that goes to the pinhole in the cover. A regular rounded o-ring of the correct size will work there fine too. We tend to disable those to prevent issues later on, all they are is a pollution device.

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