Holley carb tuning
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  1. #1
    Rods 'n' Sods Junkie SpannerPete's Avatar
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    Holley carb tuning

    Hi,

    I've got a 383 SBC with a holley 750cfm (0-3310S) and a single plane high rise manifold.

    It idles ok has loads of go when on the gas but at very light throttle is coughs/misses/judders (1400 rpm/ 30 mph just when you need it!).

    It has about 9" of vacuum at idle and a 5" power valve. Vacuum when the problem occurs is about 15", a little throttle and the vacuum drops to below 10" and the problem goes.

    I've tried upping the spring rate in the vacuum secondary and this didn't help but just gives a slight hesitation when I go on the gas.

    Not sure if it makes much difference but it's in a light car (fibreglass T) and has a lightweight flywheel.

    I'm getting to the pulling my hair out stage so any suggestions welcome.

    Thanks

    Pete

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    Exceeded sell by date Plumpcars's Avatar
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    Have you got vacuum advance?

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    Rods 'n' Sods Junkie SpannerPete's Avatar
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    Quote Originally Posted by Plumpcars View Post
    Have you got vacuum advance?
    Yes, forgot to mention that sorry.

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    Official RnS Addict WB54's Avatar
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    try adjusting the pump shot or use the next size up shooter.

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    Off the Xmas card list kapri's Avatar
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    What sort of cam in there to get only 9 " at idle ? Sound radical ? What's the quoted power band?

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    Rods 'n' Sods Junkie SpannerPete's Avatar
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    Quote Originally Posted by WB54 View Post
    try adjusting the pump shot or use the next size up shooter.
    Will this effect it at steady throttle position which is where I have my problem? I'm still trying to understand all the different parts and what they all do.

    Quote Originally Posted by kapri View Post
    What sort of cam in there to get only 9 " at idle ? Sound radical ? What's the quoted power band?
    Not sure on the exact cam but it shouldn't be too radical. It should be good from idle to 6000, but I've only got a dyno sheet from 3000 to 6000

  9. #7
    Carburation 'sucks' Roscobbc's Avatar
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    Sounds as though you may have a 'transision' problem where the carb is going from off-idle (where it runs OK I presume) to main jet where I guess you have done a plug colour test at wide open throttle on a run? (to be sure its not main jetted too weak). This 'stumble' can be a really difficult thing to tune-out of the carb - typically you MAY need to change idle bleed jetting if it has removable jets or restrict/drill-out passages if a low-spec carb. It sounds typical of a 'stiff' cam/low vacumn scenario where the carb isn't pulling enough vacumn through the venturis to atomise the fuel just off-idle. With non-adjustable idle bleeds an old trick was to drill a couple of 1/8" holes in primary butterfly to allow enough air to flow off-idle.

  10. #8
    Rods 'n' Sods Junkie SpannerPete's Avatar
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    Quote Originally Posted by Roscobbc View Post
    Sounds as though you may have a 'transision' problem where the carb is going from off-idle (where it runs OK I presume) to main jet where I guess you have done a plug colour test at wide open throttle on a run? (to be sure its not main jetted too weak). This 'stumble' can be a really difficult thing to tune-out of the carb - typically you MAY need to change idle bleed jetting if it has removable jets or restrict/drill-out passages if a low-spec carb. It sounds typical of a 'stiff' cam/low vacumn scenario where the carb isn't pulling enough vacumn through the venturis to atomise the fuel just off-idle. With non-adjustable idle bleeds an old trick was to drill a couple of 1/8" holes in primary butterfly to allow enough air to flow off-idle.
    I've not done a WOT yet but it has been set up on the dyno so I assume that should be correct. I have read about the drilling tricks but am nervous about drilling a new carb. Is the 15" of vacuum at 1400rpm not enough?

    Thanks for all the replies

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    Off the Xmas card list kapri's Avatar
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    Antons T is running a 1500-6000 cam and pulls about 18" at idle . My Pinto had a 1000-6000 cam and was pulling 21 at idle . His used to read around 16 until we sorted the vacuum secondary spring which initially had a blocked vacuum port to the vacuum secondaries .It was brand new carb out the box as well. I always tune for max vacuum at tickover by balancing timing as well as mixture and tickover screws.

    If your vacuum is too low at that 1400 cruise it could be causing the secondaries to be opening too early and making it stumble. Higher tickover vacuum should move the effect elsewhere in the rev range which can be tuned with secondary vacuum springs then.

    Wiht Antons, also a T bucket we had to go several springs lighter to get the best performance.

    Just looked back at your initial post and I'm not understanding why you start at 9 " of vacuum and it's increasing as you accelerate ? Where is the vacuum gauge reading being taken from?

  12. #10
    Exceeded sell by date Plumpcars's Avatar
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    I'd be looking at the timing curve as well. Play about with it a little. Try running without the vacuum advance as well and see if there is any affect? Where is it set to at present? What distributor and have you got stock advance springs or light?

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