Holley Carb
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Holley Carb

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    Senior Member hellnback's Avatar
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    Default Holley Carb

    My Carb 750/850 dp mech secnds doesn't have a power valve front or rear ... jets are 78/82 why would this be? It seems to work fine should I change it? 427 BBC 12.5:1 big lift solid cam
    It may be OLD ,it may be Slow ... but at least it's MOBILE !!!

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    Senior Member jockorace's Avatar
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    How's that old saying go........."If it ain't broke, don't fix it!!" If it performs well now, leave it alone. If you want max fuel economy, you could go PV in primary and jet down 6 steps or so. I'd leave it alone. JMO, Jocko

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    Old Timer LeE ss13's Avatar
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    What kind of fuel are you running? With those jets I would think Race Gas, yes/no? And how much total lead, (timing), is in it?
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    Or Seth, either one Budweiser's Avatar
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    If your idle vacuum is above 6", I'd consider taking advantage of the power valve circuit. Especially if you are not using the boat purely as a race boat.

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    gn7
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    Quote Originally Posted by LeE ss13 View Post
    What kind of fuel are you running? With those jets I would think Race Gas, yes/no?
    How can you determine that? I can't even tell from the post if its a 750 or a 850. If the jetting seems odd to you, is that for a 750 or an 850 that makes the jet size odd?



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    Yeah all the numbers are gone so without taking it off to measure.... 93 with bottle of 104 doesn't use much fuel no stumbles has big accelerater pump in back.. big squirters too. Just wondering why it would be like that setup for e85 maybe? I've just never seen one done.... . My jet Carb has one PV and 56/68 jets.
    It may be OLD ,it may be Slow ... but at least it's MOBILE !!!

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    just a ski boat with bark Carnivalride's Avatar
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    Quote Originally Posted by hellnback View Post
    My Carb 750/850 dp mech secnds doesn't have a power valve front or rear ... jets are 78/82 why would this be? It seems to work fine should I change it? 427 BBC 12.5:1 big lift solid cam

    James, the list number is usually stamped in the choke horn. My 750DPs came with 71/80 jets with a PV in the primary and a plug in the secondary. My old 850 had 82/90 jets with a PV in the primary and a plug in the secondary. This is how they were from the factory before I started tweaking on them. If I were you I'd take some plug readings and see where its at it might be fine. If the plugs are dark or black cruising I'd sure consider adding a PV on th eprimary side and rejetting.

    Loren
    Quote Originally Posted by gn7 View Post
    Carnivalride, you really need to find some other hobby. You have no talent for this boat thing

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    just a ski boat with bark Carnivalride's Avatar
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    Quote Originally Posted by hellnback View Post
    Yeah all the numbers are gone so without taking it off to measure.... 93 with bottle of 104 doesn't use much fuel no stumbles has big accelerater pump in back.. big squirters too. Just wondering why it would be like that setup for e85 maybe? I've just never seen one done.... . My jet Carb has one PV and 56/68 jets.
    My 850 had come with a 50cc pump on the secondary side and a 30cc on the primary. I ended up with it being opposite with the 50cc on the primary and 30cc on the secondary side when I was done with it though.

    An 850 will have a 1-3/4" opening at the base plate and the 750 (as well as some others) will be 1-11/16".

    If it were set up for E-85 it would be pig fat rich on gasoline.
    Quote Originally Posted by gn7 View Post
    Carnivalride, you really need to find some other hobby. You have no talent for this boat thing

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    Senior Member Deweydaddy's Avatar
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    Quote Originally Posted by Carnivalride View Post
    James, the list number is usually stamped in the choke horn. My 750DPs came with 71/80 jets with a PV in the primary and a plug in the secondary. My old 850 had 82/90 jets with a PV in the primary and a plug in the secondary. This is how they were from the factory before I started tweaking on them. If I were you I'd take some plug readings and see where its at it might be fine. If the plugs are dark or black cruising I'd sure consider adding a PV on th eprimary side and rejetting.

    Loren
    good call.....if you must play with your setup....I usually do!!!!! Lol... Good place to start is with your timing and plug readings.....could very well be more power hidden within!!!!!! Hi, Loren
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    Default Transistion

    My understanding of the P/V would describe it as a "crutch" to enable a smooth transition from Idle to the point when the main jets begin to fully flow... If the vacuum drops below a certain point when the butterflies are opened, the P/V opens and adds additional fuel to the mix to prevent a lean condition, and a resulting stumble, or "pop"... (the accelerator pump circuit/adjustment/size can also be an issue when it comes to a stumble during transition, but apparently not an issue here)
    The "norm" out of the box is usually 6.5 inches of vacuum, BUT, a properly set up, "for the combination", carb can easily be tuned with out a power valve... The down side might be a fat condition in the lower mid range due to larger primary jets, which might also play a role in fuel economy.... Another thing to remember is most of these performance Holleys were designed for automotive applications where the P/V would play a bigger role due to engine "load". Load is something we seldom see in our marine applications, therefore the removal of the P/V is perfectly understandable.. As mentioned here, if it aint broke, keep fixing it until it is....
    Ray
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    gn7
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    Quote Originally Posted by Moneypit View Post
    My understanding of the P/V would describe it as a "crutch" to enable a smooth transition from Idle to the point when the main jets begin to fully flow... If the vacuum drops below a certain point when the butterflies are opened, the P/V opens and adds additional fuel to the mix to prevent a lean condition, and a resulting stumble, or "pop"... (the accelerator pump circuit/adjustment/size can also be an issue when it comes to a stumble during transition, but apparently not an issue here)
    The "norm" out of the box is usually 6.5 inches of vacuum, BUT, a properly set up, "for the combination", carb can easily be tuned with out a power valve... The down side might be a fat condition in the lower mid range due to larger primary jets, which might also play a role in fuel economy.... Another thing to remember is most of these performance Holleys were designed for automotive applications where the P/V would play a bigger role due to engine "load". Load is something we seldom see in our marine applications, therefore the removal of the P/V is perfectly understandable.. As mentioned here, if it aint broke, keep fixing it until it is....
    Ray
    Your understanding is incorrect. Stick to pressurized sprinkler systems.

    The PV allows the carb to operate at a lean condition, usually on the primary side, during light load cruise conditions, and then richen when the engine is under higher loads. It has very little to with acceleration and virtually no effect on creating or fixing a stumble.

    It an fuel saving device that allows the carb to operate at less than peak power AF. Nothing more.

    BTW, boats are always under constant load. Why do you think that boats can't go as fast as cars with the same HP? Not to many boats can hit even 100 with a stock LS6. No problem for a Vette or Camaro.



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  14. #12
    gn7
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    Quote Originally Posted by Carnivalride View Post
    James, the list number is usually stamped in the choke horn. My 750DPs came with 71/80 jets with a PV in the primary and a plug in the secondary. My old 850 had 82/90 jets with a PV in the primary and a plug in the secondary. This is how they were from the factory before I started tweaking on them. If I were you I'd take some plug readings and see where its at it might be fine. If the plugs are dark or black cruising I'd sure consider adding a PV on th eprimary side and rejetting.

    Loren
    I'm guessing he has a 750 based on the jetting.
    Holley released 850s with a few different jettings. Later 850s even had a PV on the secondary side and a plugged primary PV, which is very odd to me. Those carbs tended to have close to square jetting and the PV was intended to only richen the AF under full load. Same thought behind the 30cc pump on the primary. Least fuel as possible when not under full throttle. Its a car thing. Not too many times in a car that the its under as much load as a boat right off idle. Boats need that shot on the primary sometimes. Probably more so in a V drive or outdrive than a jet.



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    Last edited by gn7; 02-20-2013 at 12:01 PM.

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    Powervalve is part of the power enrichment circuit. Provides additional fuel at high power settings. Some aircraft carbs had a power enrichment needle and seat. Opened mechanically at a specific throttle opening. Around 70% if I remember correctly.

    If you are OK with the power it has and fuel economy don't jack with it.
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    Led
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    750DP on a mild 468, i tried a plug in the primary and changed jets several times, but motor never liked it, so i re-installed the PV, but like the previously stated, dont jack with it

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