This was my response to an FYI'er that was having idle speed problems with his Electric Choke Holley carb.  It pretty much outlines how to modify the fast idle cam to work best on a classic Mustang.  Through many years of owning Holley carbs with electric chokes, I have found that modifying the cam by removing the second and third step will make the carb work much better and you can set the choke leaner and still have it where the carb will come off the fast idle quicker.  Nothing is more frustrating than having a carb with a mind of its own that keeps changing the idle speed on you.

First, find a full 12 volt source to connect it to, such as the terminal on the back of the alternator.  Do not use the wire from the starter solenoid or the wire to the coil.

Second,  see that little red part on the passenger side?  Push it with your thumb and hold it down as you blip the throttle slightly, with the engine idling.  If the idle drops down, then you have determined that your choke is not coming off the fast idle cam (that red thing) fast enough.  The red piece cannot be pushed to normal idle speed setting (downward) unless the throttle is open slightly.   If it is a fast idle speed cam problem, that can be caused by wrong choke heater element setting, not enough voltage to the choke heater, the fast idle screw is adjusted too far, or there is something broken/missing/dirty that is keeping the fast idle from working right. 

Moving along...
Assuming a fast idle cam issue.
1.  Loosen the three screws on the choke heater element and rotate it while holding the throttle open slightly (engine OFF this time).  If the choke butterfly moves in corelation to the rotation of the heater element, we can move on to the next step.  If it doesn't, remove the black heater element (3 screws) and reinstall, making sure the loop on the end of the internal spring is connected around the tang on the lever inside the mechanism.  Once you get that hooked up, reinstall the heater.  With the engine at room temperature, hold done the throttle slightly, then rotate the heater element until the choke is just closed tight.  Then tighten the three screws.  Retest to see if problem is fixed.  If not... here we go for step 2....

2.  OK, it must be assumed that if you held the red fast idle cam down, the carb would return to its normal idfle setting.  With the red cam all the way down, adjust the normal curb idle where you want it to idle.

3.  Next, remove the carb, drain the gas out of it, and turn it upside down.  This will let you see the fast idle cam.  The red cam has three steps on it.  Take a razor blade and remove all but the last, biggest step on the cam.  Make your cut smooth, and at about the height of the lowest part on the cam.  What you are doing is revising the cam so that when you first start the engine, the fast idle cam will be at its highest setting so the engine can idle when started cold without you having to hold your foot on the gas.  The cam is designed so that, over time as the engine warms up, it moves from the biggest step, to the middle step, and then finally the smallest step, then to no step.  Remove all but the highest step.  Then you can start the car, let it idle on fast idle for ten seconds or so, then blip the throttle and the fast idle cam will come completely off the cams so you are back at normal idle.  This works best in warm to moderate climates.  If you drive the car and it is 30 below all the time, leave the cam alone.  By removing those steps, it lets the fast idle go away quicker, and then you can adjust the choke settings as you like without having to worry about whether it is going to trip top the fast idle setting.

4.  Once you get the steps removed, close the choke butterfly so the fast idle speed screw is sitting raised up by that one step on the fast idle cam.  Adjust the screw so that you have 0.030" clearance between the curb idle speed screw (on driver side of carb) and the linkage that interfaces with the curb idle screw.  BAsically, you want the one last step on the fast idle cam to hold the throttle open 0.030" until the choke heater heats up and releases the linkage off the fast idle cam.

5.  Turn it over and bolt it back on.  Assuming all the linkage works freely and you have full 12 volts, it should work fine.

6.  If that fast idle is too fast or too slow to suit you, you can open the throttle 100% (engine OFF) and reach that fast idle screw by turning yourself upside down on the passenger fender and getting a 1/4" wrench (older Holleys used a slotted screw).  Tighten the screw to raise fast idle or loosen it to lower the fast idle.

All of that razor blade mod will allow the carb to come off fast idle much sooner, but still leave it working for initial starts, making the car much more liveable for the typical gearhead.