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Thread: CB750 Carbs

  1. #1

    Default CB750 Carbs

    So I'm building a cb750 chop with 71k1 engine, amen rigid frame, and have run into a set back. I bought a pile of parts for dirt cheap as I needed some carbs but the carbs apparently came off a cb750a and wont fit my bike. Is there anyway to make them work on a 750k or am I forced to buy new carbs? I dont mind spending money to adapt them if need be as they are in great shape and rebuilt, and I'm trying to hurry this chop along for bike week. Thanks for any help.

  2. #2
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    Found this:

    "The CB750A uses the same engine as the CB750, but detuned with lower 7.7:1 compression and smaller carburetors producing a lower output, 35.0 kW (47.0 hp)."

    Looks like the A carbs are 24mm, while the K carbs are 28mm. Will fit, just need to change the intake boots to 24mm to fit them A carbs.

    That info from here where they want to change the A carbs to the larger K carbs:
    Topic: Anyone successfully put 750K carbs on a 750A? Anything compatible?
    http://forums.sohc4.net/index.php?topic=76842.0

    Carburetor Specifications
    U.S. CB750 Models
    1969-1983
    http://www.hondachopper.com/garage/c...arb_specs.html

  3. #3

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    Thanks a ton! I found plenty of people wanting to switch to k carbs but not the other way around. So would a simple re jet and boots do the trick? Never swapped carbs before and not trying to kill power or anything.

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    It would work, but will be somewhat restricted due to the smaller size.
    A set of carb holders (Part #: 48-0761) are $119
    a set of clamps (universal) are $18
    https://www.vintagecb750.com/product...r-boots-clamps

    You will need to do some research to find out which jets will fit the A carbs, as they are a different casting than all other carbs listed on that honda chopper carb specs link above.

    jets:
    Performance Main Jet Kit(Part #: 48-2111). Kit includes a total of 16 Main Jets for Keihin Carbs('72-'78), $33
    Pilot Jet Set of 4 (Size #40). Jet Height 28mm, Outer Diameter 5.5mm., $12...do not see a size listed for the A carbs, so do they fit (28mm x 5.5mm?)
    ...or do you get a carb rebuild kit for the '70-'75 for $17 x 4, and hope that all pieces fit...I see an obvious difference in the needle jet holders on this page (a phone call maybe will confirm):
    https://www.vintagecb750.com/products/6/fuel-system

    So now you are in for about $200...depending on jets or rebuild kit, or supplier. Might get parts cheaper. Did you check around locally for carbs that fit your '71?

  5. #5
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    The best CB750 carbs are the 1969-70 version. Hands down!

    Easiest to tune and respond.
    No heavy linkages on top.
    The float bowl can be removed while on the bike with just a spring clip!
    The idle screws are marked with a arrow.
    The slides can be removed while mounted on the bike!
    Use the #42 idle jets with aftermarket air filters and aftermarket exhausts.
    I just saved you a lot of time and frustration.
    The best performance is to use the stock air filter box with the built in venturi tubes!
    You can use these carbs on any CB750!
    Last edited by Luky; 09-10-2019 at 8:36 AM.

  6. #6

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    Sounds like buying new carbs is a better option. Thanks fellas

  7. #7
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    Do yourself a favor and get a set of the early 70's Keihin carbs for your bike. Either the 657A or 657B stamped ones or "Round Tops" as people refer to them. They have the most available parts thru Honda and they also still make the boots so you don't have to deal with trying to renew old rubber parts.

  8. #8
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    Quote Originally Posted by SportsterShit View Post
    So I'm building a cb750 chop with 71k1 engine, amen rigid frame, and have run into a set back. I bought a pile of parts for dirt cheap as I needed some carbs but the carbs apparently came off a cb750a and wont fit my bike. Is there anyway to make them work on a 750k or am I forced to buy new carbs? I dont mind spending money to adapt them if need be as they are in great shape and rebuilt, and I'm trying to hurry this chop along for bike week. Thanks for any help.
    I have some early roundtops I will sell you. they will need rebuilt though. I had planned to tear them down and bead blast and rebuild them but I just havent had the time. DM me

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