Holley Carb tuning
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Holley Carb tuning

  1. #1
    Senior Member Casey629's Avatar
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    Default Holley Carb tuning

    Hi there guys

    I am writing this because I am at a loss. I would like to preface this by saying I am NOT a mechanic but do have some average knowledge of how a motor works and the mechanics involved. My father in law helped me build this and he was an old gear head from way back, but he has since pasted on so I am turning to other professionals, like you for help. I am also not looking to start any wars or have any smart ass answers or someone make fun of me or run their mouth like I have seen before.
    With that being said...
    I am trying to tune my flatbottom for the summer and need some words of encouragement.
    Motor specs:
    Chevy 454 .30 over
    Big Cam in it
    Holley 750 with a vacuum secondary
    Edelbrock intake manifold
    Rocker arms

    A few years back this thing ran like a top. The problem that I am having is it will start just fine, and once it is warm it will stay running; however the motor idles extremely rough and at about 1500 rpm(s) I have backed the idle mixture screws way out (probably to far)The curb idle screw is all the way down, but if I back it off the motor will die. When it is idling at that 1500 rpm(s) it will surge to 2000 rpm(s) and then back down and sometimes dip to 1100 rpm(s) or so. I have adjusted the float and the accelerator pump work just fine, in fact it has no hesitation or any lag when you give it some gas. I dont think I have any type of vacuum leak or anything like that. I believe it is solely in the way the carb is set up right now.
    I know that there is something that I am missing and I know it is most likely a very small issue, I just want to solve this issue.


    Any suggestions to set the idle at about 1100 rpm(s) and have a good idle without the surge?

    Any suggestion will help!

    Let me know if you have any questions or need any more information on the motor.

    Thanks for your time and advice!!!
    Last edited by Casey629; 04-03-2013 at 07:52 PM.
    "OH MY GOD! IT Works!!!" as said by my Wife Marissa on the 1st running of my flatbottom 'Miss Madison'

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  3. #2
    Senior Member SBS933's Avatar
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    Quote Originally Posted by Casey629 View Post
    Hi there guys

    I am writing this because I am at a loss. I would like to preface this by saying I am NOT a mechanic but do have some average knowledge of how a motor works and the mechanics involved. My father in law helped me build this and he was an old gear head from way back, but he has since pasted on so I am turning to other professionals, like you for help. I am also not looking to start any wars or have any smart ass answers or someone make fun of me or run their mouth like I have seen before.
    With that being said...
    I am trying to tune my flatbottom for the summer and need some words of encouragement.
    Motor specs:
    Chevy 454 .30 over
    Big Cam in it
    Holley 750 with a vacuum secondary
    Edelbrock intake manifold
    Rocker arms

    A few years back this thing ran like a top. The problem that I am having is it will start just fine, and once it is warm it will stay running; however the motor idles extremely rough and at about 1500 rpm(s) I have backed the idle mixture screws way out (probably to far)The curb idle screw is all the way down, but if I back it off the motor will die. When it is idling at that 1500 rpm(s) it will surge to 2000 rpm(s) and then back down and sometimes dip to 1100 rpm(s) or so. I have adjusted the float and the accelerator pump work just fine, in fact it has no hesitation or any lag when you give it some gas. I dont think I have any type of vacuum leak or anything like that. I believe it is solely in the way the carb is set up right now.
    I know that there is something that I am missing and I know it is most likely a very small issue, I just want to solve this issue.


    Any suggestions to set the idle at about 1100 rpm(s) and have a good idle without the surge?

    Any suggestion will help!

    Let me know if you have any questions or need any more information on the motor.

    Thanks for your time and advice!!!
    You don't think you don't have a vacuum leak or you confirmed it ? If you have done so and you think it's the carb, take it in and get it done over, then check it off the list if it still runs the same. or take it to a good boat mechanic. simple things will kick your ass. a good mech. will be cheaper than replacing things one by one.
    Believe Nothing you hear and 1/2 of what you see.

  4. #3
    Senior Member Casey629's Avatar
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    Quote Originally Posted by SBS933 View Post
    You don't think you don't have a vacuum leak or you confirmed it ? If you have done so and you think it's the carb, take it in and get it done over, then check it off the list if it still runs the same. or take it to a good boat mechanic. simple things will kick your ass. a good mech. will be cheaper than replacing things one by one.
    The reason that I don't think I have a vacuum leak is that it ran like a champ before. I assume a leak can occur over a period of time...but I hate to assume.
    "OH MY GOD! IT Works!!!" as said by my Wife Marissa on the 1st running of my flatbottom 'Miss Madison'

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    Senior Member ol guy's Avatar
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    Quote Originally Posted by Casey629 View Post
    The reason that I don't think I have a vacuum leak is that it ran like a champ before. I assume a leak can occur over a period of time...but I hate to assume.
    Could be that the needle and seats or lifting due to sinking float or needle and seat are bad and allowing the bowls to over flow? Fire it and when it starts to run bad shut it down and recheck the float bowl level.

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    You said it ran good a few years back, how long has it been since it was run last? Could be just a gummed up carb and need a rebuilt. Just my guess. Good luck.

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    Senior Member ol guy's Avatar
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    Quote Originally Posted by cubegenie View Post
    You said it ran good a few years back, how long has it been since it was run last? Could be just a gummed up carb and need a rebuilt. Just my guess. Good luck.
    Agreed!!!!

  9. #7
    Senior Member Casey629's Avatar
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    Quote Originally Posted by ol guy View Post
    Agreed!!!!
    The carb is clean as far as I can tell. I took it off the boat and took the bowls off. The jets were clean as well as the needles. I think it is just a matter of getting it set to the correct tuning.

    Another question After going back and starting it again, I noticed a small drip of gas from the accelerator pump coming the the arm area. I plan on correcting that. Would that have a huge impact on the idle?
    "OH MY GOD! IT Works!!!" as said by my Wife Marissa on the 1st running of my flatbottom 'Miss Madison'

  10. #8
    Senior Member bp298's Avatar
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    Quote Originally Posted by Casey629 View Post

    A few years back this thing ran like a top. The problem that I am having is it will start just fine, and once it is warm it will stay running; however the motor idles extremely rough and at about 1500 rpm(s) I have backed the idle mixture screws way out (probably to far)The curb idle screw is all the way down, but if I back it off the motor will die. When it is idling at that 1500 rpm(s) it will surge to 2000 rpm(s) and then back down and sometimes dip to 1100 rpm(s) or so. I have adjusted the float and the accelerator pump work just fine, in fact it has no hesitation or any lag when you give it some gas. I dont think I have any type of vacuum leak or anything like that. I believe it is solely in the way the carb is set up right now.
    so was this thing parked -some- years ago with gas in the carb? gas in the tank? it very well may be that the carb needs to be taken apart, thoroughly cleaned and blown out, gaskets and o-rings replaced. gas left in a carb can make a mess.

    mixture screws should be set 1 1/2 turns out from "bottomed" to start with. only very slight adjustments of those are necessary. when you screw the "idle" screw down, you are opening the throttle blades. basically drawing more fuel out of the transition circuit. backing the idle mixtrue screws way out, in combination with opening the throttle blades, making the engine way too rich which is probably causing it to die. what do the plugs look like after long idling? and if you've got old (bad) gas, or a lot of crud built up in the carb from just sitting, the situation is made worse.
    i'd go through the carb. make sure everything is super clean. you may not know how to do this, but there's no time like the present to learn. it's not brain surgery, really. there are tons of books on the subject. take your time, do it right.

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    Senior Member bp298's Avatar
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    Quote Originally Posted by Casey629 View Post
    The carb is clean as far as I can tell. I took it off the boat and took the bowls off. The jets were clean as well as the needles. I think it is just a matter of getting it set to the correct tuning.

    Another question After going back and starting it again, I noticed a small drip of gas from the accelerator pump coming the the arm area. I plan on correcting that. Would that have a huge impact on the idle?
    you posted while i was typing.

    gas dripping from the accelerator pump means the accelerator pump diaphram is leaking. with the carb off, just remove the 4 screws attaching the pump, remove the case and the diaphram. there's a spring between the diaphram and the bowl, which you'll need to catch. this carb will either have the little steel ball checks, or a neoprene flapper check. if it's the neoprene check, that should be replaced while it's apart. if it's the steel ball check, make sure it's nice and free and plugs the hole.

    no this will have no impact at all on idle. but that diaphram won't heal itself and needs to be replaced so it doesn't create a fire hazard.

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    Senior Member ol guy's Avatar
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    Quote Originally Posted by Casey629 View Post
    The carb is clean as far as I can tell. I took it off the boat and took the bowls off. The jets were clean as well as the needles. I think it is just a matter of getting it set to the correct tuning.

    Another question After going back and starting it again, I noticed a small drip of gas from the accelerator pump coming the the arm area. I plan on correcting that. Would that have a huge impact on the idle?
    Accelerator pump leak will not affect idle but it may be telling you that maybe you have some warped surfaces other than that. Did you pop the metering plates while you had the bowls off? Normally when a holley has sat for a winter or longer all sorts of gremlins pop up in carbs. Should be nothing more than a tear down regasket, clean and reassemble. Good luck.

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    gn7
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    Quote Originally Posted by ol guy View Post
    Accelerator pump leak will not affect idle but it may be telling you that maybe you have some warped surfaces other than that. Did you pop the metering plates while you had the bowls off? Normally when a holley has sat for a winter or longer all sorts of gremlins pop up in carbs. Should be nothing more than a tear down regasket, clean and reassemble. Good luck.

    And the cheaper brown Holley gaskets have a bad habit of shrinking when the sit long periods with no fuel. Messes up the idle big time.
    I have had a bowl gasket leak that you would swear was a accelerator pump leak. Real slow, and it would capillary under the bowl and drip off the accelerater pump.



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    Senior Member Casey629's Avatar
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    Quote Originally Posted by bp298 View Post
    so was this thing parked -some- years ago with gas in the carb? gas in the tank? it very well may be that the carb needs to be taken apart, thoroughly cleaned and blown out, gaskets and o-rings replaced. gas left in a carb can make a mess.

    mixture screws should be set 1 1/2 turns out from "bottomed" to start with. only very slight adjustments of those are necessary. when you screw the "idle" screw down, you are opening the throttle blades. basically drawing more fuel out of the transition circuit. backing the idle mixtrue screws way out, in combination with opening the throttle blades, making the engine way too rich which is probably causing it to die. what do the plugs look like after long idling? and if you've got old (bad) gas, or a lot of crud built up in the carb from just sitting, the situation is made worse.
    i'd go through the carb. make sure everything is super clean. you may not know how to do this, but there's no time like the present to learn. it's not brain surgery, really. there are tons of books on the subject. take your time, do it right.
    I just looked at the plugs, They seem a bit rich. I am going to pull the carb down tomorrow and go through it again. The accelerator pump is neoprene and I believe it just needs to be tighten up and put back together. There was no gas in the tank or the carb while it was sitting. It only sat for less than a year.
    i will let you know how everything looks tomorrow. I am going to fire it back up and try to get the idle correct.
    Another question: will the idle be affected by adding water to the motor? I have a hose I hook up.

    Thanks for everyone's advice so far! I really appreciate it!
    Last edited by Casey629; 04-03-2013 at 10:52 PM.
    "OH MY GOD! IT Works!!!" as said by my Wife Marissa on the 1st running of my flatbottom 'Miss Madison'

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    gn7
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    Quote Originally Posted by Casey629 View Post
    I just looked at the plugs, They seem a bit rich. I am going to pull the carb down tomorrow and go through it again. The accelerator pump is neoprene and I believe it just needs to be tighten up and put back together. There was no gas in the tank or the carb while it was sitting. It only sat for less than a year.
    i will let you know how everything looks tomorrow. I am going to fire it back up and try to get the idle correct.
    Another question: will the idle be affected by adding water to the motor? I have a hose I hook up.

    Thanks for everyone's advice so far! I really appreciate it!
    Having the engine warm before messing with the idle is a plus. But it doesn't effect the idle to the extent you are describing.



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    Quote Originally Posted by Casey629 View Post
    I just looked at the plugs, They seem a bit rich. I am going to pull the carb down tomorrow and go through it again. The accelerator pump is neoprene and I believe it just needs to be tighten up and put back together. There was no gas in the tank or the carb while it was sitting. It only sat for less than a year.
    i will let you know how everything looks tomorrow. I am going to fire it back up and try to get the idle correct.
    Another question: will the idle be affected by adding water to the motor? I have a hose I hook up.

    Thanks for everyone's advice so far! I really appreciate it!
    Warning!!!! Don't just try to tighten the accell pump, take it off and check it for flat! Also don't waste your'e time with anything other than holley blue gaskets. Also check the o-rings on the needle and seat, the rip and you can set the float level all day long and with a bad o-ring the level will just keep creeping up. Good luck

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