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  1. #11
    Stroker Kustom Jim's Avatar
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    LightbulbSingle planes do that don't they?

    So it runs uneven when you're cruising at a steady 30?
    And not when you boot it when you're doing 30!

    Dunno much meself but trying to pin down problem

    I've followed this guide and sat and watched the subtleties of how the gauge needle moves and what it's telling you.
    Vacuum gauge, the wonder tool

    Or, if it's been dyno'd and was fine mebbes the dizzy's loose/moved.

    cheers
    Jimbo
    "I'm not going to take this defeatist attitude and listen to all this crap any more from all these people who have nothing except doomsday to predict."
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  3. #12
    Official RnS Addict WB54's Avatar
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    Quote Originally Posted by SpannerPete View Post
    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.



    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

    sorry read you wrong, is it a stumble when you floor it from idle or stumbling at low rpm cruise speeds ? Does it foul plugs, smell rich, backfire through carb or exhaust, is it a misfire dropping cylinders etc.

  4. #13
    Off the Xmas card list kapri's Avatar
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    Click on scenario 10 on this link, may be the answer and would certainly give the sympthoms you describe?

    How to Use and Interpret a Vacuum Gauge

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  6. #14
    Rods 'n' Sods Junkie SpannerPete's Avatar
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    Quote Originally Posted by Plumpcars View Post
    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?
    It's got a Engine Works USA, Outlaw series billet alum. race distributor with Megafire iginition. Timing set to about 14-16 deg at about 800 rpm. not sure about the springs

    Quote Originally Posted by Kustom Jim View Post
    So it runs uneven when you're cruising at a steady 30?
    And not when you boot it when you're doing 30!

    Dunno much meself but trying to pin down problem

    I've followed this guide and sat and watched the subtleties of how the gauge needle moves and what it's telling you.
    Vacuum gauge, the wonder tool

    Or, if it's been dyno'd and was fine mebbes the dizzy's loose/moved.

    cheers
    Jimbo
    Yep you got it. Your the first one to mention about single plane manifolds doing that, I did wonder about it too. There is some bad reports on the web but also some good ones as well.

    Interesting link

    Quote Originally Posted by WB54 View Post
    sorry read you wrong, is it a stumble when you floor it from idle or stumbling at low rpm cruise speeds ? Does it foul plugs, smell rich, backfire through carb or exhaust, is it a misfire dropping cylinders etc.
    Doesn't foul plugs but they do look rich (black), doesn't backfire but it is almost like a misfire/cough


    Quote Originally Posted by kapri View Post
    Click on scenario 10 on this link, may be the answer and would certainly give the sympthoms you describe?

    How to Use and Interpret a Vacuum Gauge
    Wow, never seen that before. I think it's more like scenario 3 but the vacuum does seem low but maybe it's just the cam.

    Thanks for all the suggestions.

  7. #15
    Carburation 'sucks' Roscobbc's Avatar
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    Quote Originally Posted by SpannerPete View Post
    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
    Dyno results are not always good enough for 'everyday use' - they are typically used to achieve max power figures - same when I had my engine dyno'd, even though it was all optimised for max power (528 bhp) it was actually too weak under normal street conditions and needed primaries a few sizes larger. Same with idle and transision off idle, dyno properly can't replicate real world usage. My set-up shows 9/10 hg on idle and I have changed idle bleeds several times to try an cure stumble at around 1100 rpm coming-off idle but have never completely cured it, accepting that I'll cruise in town at 1400/1500 rpm in 3rd gear.

  8. #16
    Official RnS Addict WB54's Avatar
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    Quote Originally Posted by Roscobbc View Post
    Dyno results are not always good enough for 'everyday use' - they are typically used to achieve max power figures - same when I had my engine dyno'd, even though it was all optimised for max power (528 bhp) it was actually too weak under normal street conditions and needed primaries a few sizes larger. Same with idle and transision off idle, dyno properly can't replicate real world usage. My set-up shows 9/10 hg on idle and I have changed idle bleeds several times to try an cure stumble at around 1100 rpm coming-off idle but have never completely cured it, accepting that I'll cruise in town at 1400/1500 rpm in 3rd gear.

    have a look at the position of the transition slots relative to the throttle blades. The amount of slot below the blades varies among castings. You may find that extending the slot helps. Assuming you're using a 3 circuit dominator you can also reconfigure 4150 metering blocks to give you jetting and bleed control over the transion slot.

  9. #17
    Rods 'n' Sods Junkie SpannerPete's Avatar
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    Quote Originally Posted by Roscobbc View Post
    Dyno results are not always good enough for 'everyday use' - they are typically used to achieve max power figures - same when I had my engine dyno'd, even though it was all optimised for max power (528 bhp) it was actually too weak under normal street conditions and needed primaries a few sizes larger. Same with idle and transision off idle, dyno properly can't replicate real world usage. My set-up shows 9/10 hg on idle and I have changed idle bleeds several times to try an cure stumble at around 1100 rpm coming-off idle but have never completely cured it, accepting that I'll cruise in town at 1400/1500 rpm in 3rd gear.
    I was told it would need a bit of tuning on the road.

    Have you got a single plane manifold? Is this a major factor or is it the cam that causes the problem?

    My only other experience with V8's is with a Rover V8 and I just rebuilt the engine, slapped on a new carb and it runs like a train

  10. #18
    I'm Not Jed Clampett stueeee's Avatar
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    Quote Originally Posted by WB54 View Post
    have a look at the position of the transition slots relative to the throttle blades. The amount of slot below the blades varies among castings. You may find that extending the slot helps. Assuming you're using a 3 circuit dominator you can also reconfigure 4150 metering blocks to give you jetting and bleed control over the transion slot.
    If your problem is with transition, taking the carb off without touching the tickover screw and looking at the position of the butterfly vs. the idle feed slot is a good indication. You should only be able to see a small amount of the vertical slot visible below the butterfly. This slot does the transition from the fuel bleed/air bleed (according to which Holley model you have) to the unrestricted pilot jet before the main system in the top of the carb starts working. If too much of the slot is exposed at tickover, you lose the transition fueling and the motor runs like crap until the main system gets going.
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  11. #19
    Carburation 'sucks' Roscobbc's Avatar
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    Quote Originally Posted by SpannerPete View Post
    I was told it would need a bit of tuning on the road.

    Have you got a single plane manifold? Is this a major factor or is it the cam that causes the problem?

    My only other experience with V8's is with a Rover V8 and I just rebuilt the engine, slapped on a new carb and it runs like a train
    I am using a 'bracket race' cam, power band 2500 to 6500 - I specifically chose the Edelbrock RPM Air Gap inlet which is a dual plane - the reason? with a street use BB Chevy its the best choice across the range and looses out only to Victor single plane at 6000 rpm plus (and it meant that port velocity would be high at low rpm = low rpm streetability). A single plane inlet will work fine on a big cube engine at lower rpm where you have bundles of torque - but less so on smaller cubes, heavy vehicle and cars with low numerical axle ratios.

  12. #20
    Carburation 'sucks' Roscobbc's Avatar
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    Quote Originally Posted by WB54 View Post
    have a look at the position of the transition slots relative to the throttle blades. The amount of slot below the blades varies among castings. You may find that extending the slot helps. Assuming you're using a 3 circuit dominator you can also reconfigure 4150 metering blocks to give you jetting and bleed control over the transion slot.
    WB54 - I'm using a ProForm 950 DP - good call about opening-out the 'slots' - makes sense - is that opening-out vertically? - would you need to mill the slot or is it do-able with a 'swiss' file?

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