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Old 08-29-2003, 10:29 AM
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Idle restriction

I am having trouble adjusting the idle on my Holley 4150. I have read that you hook up a vacuum gauge and adjust the screws to get the most vacuum. Problem is that adjusting the screws seems to have no effect. (Unless I turn them in completely which stalls the car).

I have examined the carb and found that the transfer slot was exposed way to much so I opened up the secondarys more and drilled holes in the throttle plates. But I still cant seem to get the idle screws to make much of a difference.

To me the car seems to be running lean on idle. Is it a good idea to drill out the idle feed restrictors on the metering block? I hate the idea of punching holes in my carb but I am running out of options.

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Old 08-29-2003, 12:58 PM
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First off, what`s your combo? what cam, compression, intake, cubic inches and etc.
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Old 08-29-2003, 01:02 PM
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400 sbc
comp cam xe274h 0.490" lift
holley 700 cfm 4150 manual choke
10:1 cr
weiand stealth dual plane intake
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Old 08-29-2003, 01:13 PM
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if the transfer slot is opened too much either you have not enough ignition timing or a bad vacuum leak, causeing the butterflies to be open too much to get it to idle. so I would say first to check the timing with a timing light and see what it`s set at. you need around 12 degree`s initial timing. after your sure the timing is set, then look for vacuum leaks, if none are found, next check the fuel pressure, if it`s excessive, it`ll cause a problem similiar to yours. the cam your running isn`t very big so you shouldn`t need the aide of the secondary`s to get it to idle. best way to set the idle is with a vacuum gauge, but keep in mind also, wait until the engine is at operating temp with the air cleaner on when setting the idle mixture, and go slow as you turn them, as too much can make you overshoot where you need to be, only turn them 1/8 to 1/4 turn at a time, then give the car some throttle between adjustments on each side.
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Old 08-29-2003, 01:20 PM
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how much fuel pressure would you consider too much? It is running ~ 6-7 psi.
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Old 08-29-2003, 01:32 PM
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some say Holley`s and Demon`s can handle up to 9 PSI, well, my Demon can`t, if I go over 6 PSI it starts giving me problems idle wise, too much fuel pressure can cause it to pour out of the idle slots at idle, and it won`t be visible by the eye being there not exposed due to the butterflies. lower your pressure down to about 4 - 5 psi, I shoot for 4 1/2 mostly, and it works out okay for me.
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Old 08-29-2003, 01:33 PM
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I don't know about those Holleys, but QJets and most Rochesters can run on 3 PSI. It's not pressure, it's usually volume that causes a problem. You have to fill the bowl, especially if the secondaries are opening.

But back to the vacuum. You can change the spring and tension to fit the needs of your setup.
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Old 08-29-2003, 01:34 PM
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Thanks, I'll try that.
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Old 08-31-2003, 06:37 PM
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I just got my emmisions done and it is showing 0.01% for CO at idle and 1.47% at driving conditions. The way I interput this is too lean at idle and the jetting seems right on.

So this takes me back to my original question, should I drill my idle restrictors in my metering block or is there possible some other restriction in my idle circuit.
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Old 08-31-2003, 07:40 PM
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If the secondarys are opened to much that will lean it at idle no extra fuel no idle circuits in the secondaries.
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Old 08-31-2003, 08:04 PM
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With radical cams the idle vacuum may not change much, you may have to adjust by how it launches and how it responds to the throttle. The idle screws are effective just not responsive if I read this right. If you have a digital tach like one of those built into a timing light you can use that to adjust idle mixture.

Have you tried turning down the idle speed to the point of where it might stall and then adjusting, this usually works for me and it gets the screws into their most effective rpm range.

What kind of vacuum are you pulling?
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