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  1. #1
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    Default Vacuum bounce at idle

    Im in the process of setting my Holley idle mixture screws on my new 350 and connected a vacuum gauge to the vac port on the holley base plate and noticed the reading is rapidly fluctuating between 10 and 13 on the gauge, question is could this be from the cam size or do I have a problem?
    (Comp Xtreme XE274H 230/236 @50)
    The vac reading seems to steady with revs.

    Everything is new and still running the engine in and don't notice anything bad when driving just curious as what it could be.

    I can give more engine specs if relevant

    Any thoughts?

  2. #2
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    I think most likely cause is a vacuum leak or valve adjustment [ valves not fully closing because of wrong lifter pre-load adjustment ]. Also check the PCV. Block the PCV. If vac is then steady, PCV is pulsating from low vac of this cam & a PCV with a softer spring is needed.

  3. #3
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    Ok well the PCV valve check will be easy when I get a chance (I'm away for a week)
    After that might do a compression test first to maybe help narrow down which valves to concentrate on.

    I've never used a vac gauge before so I have no idea if a erratic needle is normal or not but sound like it's not so will look into it further for sure

  4. #4
    Senior Member LXCHEV's Avatar
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    My 383 has just a tiny bit more cam than yours (236/242 @ 0.050) - and the vacuum readings definitely jump around a little bit at idle. As soon as the revs are bought up (even just a small amount) the readings are super smooth - it's just at idle where they fluctuate. I've always put it down to the cam just being a little grumpy.

    Still a good idea to run through all your mechanical checks as mentioned above - I'd be keen to hear if you find anything.

  5. #5
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    hi
    LXchev
    How much vacuum are you making at idle . ?? eg 8-10 10--12 or is the swing bigger than that.
    What is your idle speed ??
    thanks

  6. #6
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    Nice, if I was to build again I'd think I'd choose a bigger cam also. Still, happy with this one.
    Yeah will report back when I get a chance to check a few things

  7. #7
    Senior Member BasicQ's Avatar
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    Default Vacuum bounce at idle

    This is my vacuum gauge, 383 sbc with cam int 245* @.050”. Engine warmed up in gear. Also shows idle AFR in lambda scale (about 13.1-13.4:1). If you can’t work your mix screws to similar then maybe it is one of Geoff’s suggestions.

    What carby?

    https://youtu.be/3-iTjoBcgrk




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    Last edited by BasicQ; 07-04-2021 at 07:03 PM.

  8. #8
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    At the moment I have a new 600 Holley vac secondary that set straight out of the box and plan on keeping it until the engines run in then look at buying a 750 vac Street HP (HO0-82750SA).
    Might be over kill but it has a few features I like

  9. #9
    Senior Member BasicQ's Avatar
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    The 600 should allow for a good idle and the 750 a good choice if you prefer vac sec.

    Where is your initial ignition timing at?


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  10. #10
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    Total timing is 35 and from memory the initial is around 18

  11. #11
    Senior Member LXCHEV's Avatar
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    swampy - I only make a pathetic 6" of vacuum at idle. A bit sad really because there's nothing at all wild about my engine. It's been a while since I've had the gauge on it - but from memory, the fluctuation was similar to the video posted by BasicQ - probably a good 1 increment (for example 5.5" to 6.5"). And for this reason, I've set my idle speed just under 1,000rpm rather than lower at say 800rpm which I've had in the past with a richer idle. Just keeps the car a little happier (last time on the dyno when checking this new carb of mine, we tried to get the idle as lean as possible).

    I'm overdue to revisit all this actually as I'm still not happy with my idle and have a significant hesitation just off idle. Need to install the MTX-L Plus air:fuel gauge that 'Santa' dropped under the Christmas tree last year - then I can play.

  12. #12
    Senior Member LXCHEV's Avatar
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    I should also add - my lack of ignition timing control also seems to contributes to my weak idle vacuum. I'm still running a 20+ year old ICE Compu-Curve box and to keep my total timing around 32-34 degrees, I can't run any more than 20ish initial. If I do run a few more degrees of initial the car feels heaps better (idle quality, idle vacuum and responsiveness). So it's just yet another thing I'd like to upgrade one day (but cash-flow says otherwise).

  13. #13
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    Quote Originally Posted by LXCHEV View Post
    I'm overdue to revisit all this actually as I'm still not happy with my idle and have a significant hesitation just off idle.
    I wouldn't say mine is significant but I do have similar hesitation off idle especially when cold, I initially assumed this was the cam but most likely the carby.
    To get a decent idle I have to set the idle adjustment screw far enough to expose the transfer slots in the primarys approx 3mm so could be an issue there. Apparently the fix is to drill a small hole in the primary butterfly's to allow airflow. Im yet to try this

  14. #14
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    hi
    LXchev
    I`m building a 400 with single plain and 230/236 HR with a Q/fuel SS780 vac sec .Supposedly give 10--12 inch vacuum. Your results have me concerned .

    The reason I went 400 was to get max street ability .
    The s/plain the Weiand 7530 is a small single plain and was once offered with a vac increasing plenum divider plate !!

    What manifold are u running ??
    An Ede RPM for a 383 is suppose to work well [still breathe at the top end ] . By comparison a 400 finds an Rpm manifold a little restrictive at thee top end . A 410+ s/p all the way .

    OP
    78Hz
    By keeping parts a little conservative for the cubic inches driveability will be maximised.

  15. #15
    Senior Member LXCHEV's Avatar
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    I’m running an Edelbrock RPM air-gap dual plane on the 383.

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