Setting idle on Holley 4160. Clarification needed. - Vintage Mustang Forums

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post #1 of 5 (permalink) Old 05-10-2004, 02:53 PM Thread Starter
HoosierBuddy's Avatar
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Hey guys,

I am a bit confused about the primary idle adjustment screw on a Holley 4160 AND the idle screws on the sides of the carb.

Here's the deal. My car was/is/always has been very rich at idle. In order to combat this, I readjusted the idle metering screws on the side of the carb from 2 turns open to 1 turn open. The car will actually barely idle with the primary idle screw (the one that acts as a throttle stop and advances the throttle as you turn it in) completely backed out...HOWEVER it really chugs and just seems like it's starved for air.

SO, I adjusted the primary screw in about 1 turn and that makes it idle better.

My question is this. When you adjust that primary screw from 0 to 1 turn are you JUST giving the car more air or are you also giving the car more fuel? As I'm trying to combat this rich idle condition I'm wondering which is correct.


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post #2 of 5 (permalink) Old 05-10-2004, 03:06 PM
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I don't know what you mean but I think you're kinda doing it backwards. You need to set the idle mixture screws(on the sides) with a vacuum gauge and/or tachometer to verify optimum idle mixture. Then your idle might be going real fast or might be about to die so you set the idle speed after that.

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post #3 of 5 (permalink) Old 05-10-2004, 03:07 PM
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The air/fuel mixture screws adjust the fuel/air mixture at idle.

The idle speed screw on the linkage opens and closes the butterfly for optimum RPM.

To properly set the idle, you have to endure several iterations of going back and forth between these adjustments until you get it right. The air/fuel mixture screws are key to setting the idle. They must be set optimally. Then, the idle speed screw is used to adjust the RPM to specs.

The pitfall is that if the idle speed screw is turned in too much, the butterfly is opened to where the idle circuit is effectively bypassed. When that happens, the adjustment of air/fuel mixture screws doesn't affect the idle since the vacuum now feeds the engine through the main jets. You've seen posts on the forum where someone says they can turn the mixture screws all the way in and it still idles too fast. This is because the idle speed adjustment screw is in too far and opening the butterflies to where there is no idle circuit, and the fuel is coming through the primaries.

Hope this helps some. Good Luck!


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post #4 of 5 (permalink) Old 05-10-2004, 03:07 PM
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Usually turning the idle mixture screw CC, that richens the idle mixture. I know at one time, some of Holley's emmission calibrated carbs, the idle miixture screw actually increased air and not fuel. The fuel was a fixed amount.

What time of cam are you using? With hot cams, raising the idle speed to get the engine idle by increasing idle speed, you'll move the throttle plates past the transition, rendering idle miixture circuit useless. The trick is to drill holes in the idle plate to allow the idle plates to cover the circuits to allow adjustment. I'm not saying you'll need to do this though.


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post #5 of 5 (permalink) Old 05-11-2004, 06:49 AM
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As Tom said there are two types of 4160 carbs. I have got a list 9210 which is for 318-360 77 Mopar Van, it has "reverse" mixture screws. I don't think it is even possible to get too rich idle with these emission 4160s.

On the assumption that you have a normal 4160, you lean the idle by turning mixture screws in. One and half turn is the general starting point. If you can't get it any leaner by turning the screws in, you might have the dreaded power valve leak.

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