Carb size question
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Carb size question

  1. #1
    Senior Member Itsahobby's Avatar
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    Default Carb size question

    Hi all,
    Time for a new carb and not sure of the size I should get. I've check a bunch of the online calculators and get different numbers, even when using the same VE, engine size and RPM. Also, I would think the choice would be different for a boat.
    So here's the engine specs:
    Current carb: Barry Grant Silver Claw 850, about 20 years old. If I remember correctly, they flowed more than advertised.
    I have to confess, I never played around with any changes and this carb, even though I really wasn't happy with the way it ran.
    468 CI
    9.5-1 compression, according to the builder
    ISKY Z-77 cam .342 cam lift, Valve lift .590 Duration 310, Flat tappet
    Heads have be ported and gasket match, along with the intake
    Intake is a Chevy Aluminum, I think they were made by Weiand dual plane. The divider has be machined down, about 3/8 of an inch.
    IMCO exhaust.
    Redline approx. 6500 rpm

    I'm guessing stick with a 850 CFM Double pumper carb with Mechanical secondaries?
    Thanks for your input.
    --Jerry
    Last edited by Itsahobby; 09-16-2013 at 04:40 PM.

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  3. #2
    Senior Member ol guy's Avatar
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    First off What heads? second would be in 20 years of service has that carb ever been gone thru? I imagine if the carb has never been touched it probably has more issues than Playboy! M

  4. #3
    Senior Member Itsahobby's Avatar
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    It was sent back to Barry Grant to get rebuilt in 2002, they sent it to GPT. Yes, it's toast.
    They heads are Chevy 990 rectangular port.
    --Jerry

    Quote Originally Posted by ol guy View Post
    First off What heads? second would be in 20 years of service has that carb ever been gone thru? I imagine if the carb has never been touched it probably has more issues than Playboy! M

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  6. #4
    Senior Member ol guy's Avatar
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    Quote Originally Posted by Itsahobby View Post
    It was sent back to Barry Grant to get rebuilt in 2002, they sent it to GPT. Yes, it's toast.
    They heads are Chevy 990 rectangular port.
    --Jerry
    With the engine you list,cam,heads and manifold that carb done right should be fine. Over 10 seasons without alittle care you are lucky it runs so well! You must take care of the boat well good job!!!!! M

  7. #5
    Senior Member Itsahobby's Avatar
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    It's not running well and I'm not sure if it was the best choice for my engine and was asking for some input on carb size.
    Sorry if I wasn't clear.

    The carb is pretty worn out and needs more attention than it is worth.
    Thanks,
    Jerry

    Quote Originally Posted by ol guy View Post
    With the engine you list,cam,heads and manifold that carb done right should be fine. Over 10 seasons without alittle care you are lucky it runs so well! You must take care of the boat well good job!!!!! M

  8. #6
    gn7
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    I would either send the carb of to C&J engineering, and it will come back looking better than the day BG built it, or just get a new 4781 and sell the BG on E bay. Somebody will grab it.

    C & J Engineering



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  9. #7
    Senior Member Itsahobby's Avatar
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    Default How about the 9381??

    I checked the holley site and there is another 850 carb without the choke mechanism, it is a 9381. I've not run a choke on my boats in the past. From looking at it and the specs, it looks like the same carb without a choke.
    Although I could just remove the choke off the 4781.

    Thanks so much,
    Jerry

    Quote Originally Posted by gn7 View Post
    I would either send the carb of to C&J engineering, and it will come back looking better than the day BG built it, or just get a new 4781 and sell the BG on E bay. Somebody will grab it.

    C & J Engineering
    Last edited by Itsahobby; 09-16-2013 at 06:25 PM.

  10. #8
    gn7
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    Quote Originally Posted by Itsahobby View Post
    I check the holley site and there is another 850 carb without the choke mechanism, it is a 9381. I've not run a choke on my boats in the past. From looking at it and the specs, it looks like the same carb without a choke.
    Although I could just remove the choke off the 4781.

    Thanks so much,
    Jerry
    The 9381 has annular boosters and the 4781 has down leg. The annular booster can have a little better throttle response for coming off corners in a dirt car and may have slightly better down low manners than the 4781. But the annular boosters tend to rob a little off the top due to slightly reducing the WOT CFM



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    Default Carb Size

    The basic formula for carb size is - Engine CID x Maximum RPM divided by 3456 = CFM needed.

  12. #10
    Senior Member Itsahobby's Avatar
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    Ahh, good to know.
    Thanks
    --Jerry

    Quote Originally Posted by gn7 View Post
    The 9381 has annular boosters and the 4781 has down leg. The annular booster can have a little better throttle response for coming off corners in a dirt car and may have slightly better down low manners than the 4781. But the annular boosters tend to rob a little off the top due to slightly reducing the WOT CFM

  13. #11
    gn7
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    Quote Originally Posted by Firefish View Post
    The basic formula for carb size is - Engine CID x Maximum RPM divided by 3456 = CFM needed.
    In knew those 2 dominators were over carbing my shit



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    Quote Originally Posted by gn7 View Post
    In knew those 2 dominators were over carbing my shit
    hahaha...thats funny, a lot of folks around here are probably over carb'd
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    Senior Member Itsahobby's Avatar
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    Yep, that's why I was asking. Although my plugs are always nice and brown, but I have an mental issue of always second guessing myself. My second guess is usually a mistake. lol

    I got everything from 600 cfm to 1000 from the online calculators.

    Quote Originally Posted by RPB View Post
    hahaha...thats funny, a lot of folks around here are probably over carb'd

  16. #14
    gn7
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    Quote Originally Posted by Itsahobby View Post
    Yep, that's why I was asking. Although my plugs are always nice and brown, but I have an mental issue of always second guessing myself. My second guess is usually a mistake. lol

    I got everything from 600 cfm to 1000 from the online calculators.
    An 850 is fine. Either the 9381 or the 4781 will work fine. With a mild 454, I doubt you could tell the difference between the 2.



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