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  1. #1
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    Default Holley IFR or IAB...Which to change?

    Which of these should I modify for a 240@50 cammed 308 with a stinky idle?

  2. #2
    Senior Member BasicQ's Avatar
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    Default Holley IFR or IAB...Which to change?

    Quote Originally Posted by 71rocket View Post
    Which of these should I modify for a 240@50 cammed 308 with a stinky idle?
    First things first.

    Ignition timing?

    Then, what carb?


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    Last edited by BasicQ; 21-10-2021 at 09:42 PM.

  3. #3
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    hi


    NB as above first

    fuel curve

    horizontal flat line go up --down = main jet

    angle line either up hill or down hill = low end /low speed AB ...... top end up/down HSB

    emulsion jets if the curve starts normal,, raises or dips in the curve finishes normal

    IFR can effect part throttle as well
    LSB effects more than just idle

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    Pandoras box....been there
    Chris

    68 Camaro 632 BBC Pro-touring

    Daily driver - 1961 Buick Bubbletop - not much horsepower, but smiles for miles

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    Quote Originally Posted by swampy View Post
    hi


    NB as above first

    fuel curve

    horizontal flat line go up --down = main jet

    angle line either up hill or down hill = low end /low speed AB ...... top end up/down HSB

    emulsion jets if the curve starts normal,, raises or dips in the curve finishes normal

    IFR can effect part throttle as well
    LSB effects more than just idle
    ....what the???
    Story is,650 avenger with quickfuel primary block. The engine ran rough when gently revved to 3k in neutral with idle screws 3/4 out(Vizard test),so I reduced IFR from 28 to 25 thou and now it builds revs smoothly(neutral),starts easier,smells better and now have all four screws at 1 turn out but it feels lazy when driving.
    Soooo,should I have enlarged the air bleed instead?
    I understand the fuel jet has more effect early on,air bleed later on.

  6. #6
    Do you ever leave? EH179's Avatar
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    Considering the IFR is 28 originally, yes, i would have looked at IAB first. Probably go up to .080" and test.

    With a 240 @ .050 cam in a 308, the overlap will spew unburnt fuel out. Your ignition timing and plug heat range are important here.

    What is the carb model?

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    I am assuming a hyd lifter cam & a cam that size is going to have some amount of stinky idle......
    Nature of the beast.

    [1] The t/blades are probably open too far, exposing too much transfer slot. Result is smelly idle & mixture screws have little effect.
    [2] A 240 @ 050 cam IS going to be lazy at lower rpms with a 308. It would not feel lazy in a 540. No amount of tuning is going to fix that.
    [3] You will probably need 30-40* of timing at idle for that size cam. [ yes, 30-40*!! ]. Idle will be smoother & higher, allowing t/blades to be closed more. But probably not enough to prevent too much T slot being exposed. Fix then is add bypass air by drilling t/blades if carb does not have adjustable bypass air.

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    85670-1 ,NGK 6 plugs, 20-34 timing running man vac advance. Idle speed screw 3/4 turn out then opened sec blades with no slot showing.
    On cold starts it sounds like its dropping cylinders for about first 30 seconds then cleans up.....maybe the 25 thou IFR.

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    internet fail.
    Last edited by guy incognito; 23-10-2021 at 07:09 PM.

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    IFR is a coarse adjustment, IAB fine.

    Cold start should run rough as **** until there is some heat in the manifold. If it runs nice cold, its too rich.

    If the carb has a PCV port, it will be a pig when you block it off. Carbs with PCV are designed to work with a big air leak at idle.

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    more shenanigans.

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    Plugs should be a 5 heat range.

    Is timing steady at idle with MVA connected [ congrats using MVA, smart move ]?

    What about the T slot position on primaries?

  13. #13
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    Gone back to 28thou IFR and open IAB to 82thou.....seems better but haven't driven yet.
    Barely any slot showing,it idles smoother with primaries near shut then secs open to suit.
    I tried a 5 plug but they came out white so went back to 6 (light tan)
    I do have the PCV port blocked,currently running off the carby spacer plate.
    Thanks a bunch.

  14. #14
    Senior Member BasicQ's Avatar
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    Default Holley IFR or IAB...Which to change?

    .028 IFR and .081 iab pri and sec?


    Sent from my iPad using Tapatalk

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    Quote Originally Posted by BasicQ View Post
    .028” IFR and .081” iab pri and sec?


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    Only primaries.
    Just did a cold start...big improvement. Idles easier,smells better.

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