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I have a Holley 4160 600 CFM carb with electric choke. I've been doing some reading on how it all works and I have a questions.

What is the main difference between the fast idle speed screw and the curb idle speed screw?

Does the fast idle speed screw control the idle when the choke is closed and the curb idle speed screw control the idle when the choke is open?

Also, how do you set the fast idle speed screw?
 
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What is the main difference between the fast idle speed screw and the curb idle speed screw?
The fast idle sets the idle speed when the choke is activated. The curb idle sets the idle speed when the choke is off.

Also, how do you set the fast idle speed screw?
With difficulty. As I recall, the screw is at an odd angle on the choke assembly. Preferably engine cold, choke on. Tighten or loosen the screw as needed to achieve the specified idle speed. You may need to turn the choke cap to full rich to get it to stay on long enough.
 

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Close your choke blade and then open the throttle. You will see the fast idle cam rotate to where it raises the idle speed. Now open the choke plate and move the throttle and you will see the fast idle cam rotate out of the way so that when you close the throttle you are back to your warm idle rpm.
 

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Before you adjust fast idle, check to see where it is now. On a cold engine you press the throttle to "set" the fast Idle cam then start the car. If the car won't stay running at that rpm, turn in the fast idle screw. Conversely, if it is running WAY too fast, turn the screw out (counterclockwise). When your choke coil heats up, you press the throttle to drop the idle rpm. There are several steps on the fast idle cam so your rpms may not drop down to the hot idle speed on the first try but will when the choke coil is at full temperature.
 
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