I have a 1985 F150 , original everything, auto transmission that’s been in the family forever that I’ve been restoring. Ended up replacing the original carb due to a lack of power and hard erratic idle issues (it sat for over a year and a half at the body shop during all the work and got infrequent time running).
I Placed a new carb on and now it runs great at speed, but at idle surges between a very low idle and a normal idle and then will die. If I keep the idle high by tightening up the speed control chain it runs great, just idles way to fast. Checking the carb I find the part I’ve shown in the attached pictures to cause the plunger to pulse in and out which affects my idle from normal to dying. If the idle is set high using the speed control Chain the truck runs great on the highway, just can’t really drive it far in stop and go as I don’t want to.
In the first picture I have the arrow pointing to the solenoid??that seems to cause the plunger I show in the second picture to pulse causing the idle to go up and down and eventually die.
Any help or suggestions would be greatly appreciated.
I’ve checked and changed any questionable hoses, new gaskets, new plugs, wires and fuel pump and filter

 

You have a 300 six? I believe you do looking at the carb. Your engine has a complex little understood computer control system on it called EECIV and TFI ignition. What I would do first is unplug that device, set the idle, and see how it runs.

 

The "stepper" motor is controlled by the computer, which receives input from several sensors. What Franklin2 has suggested is good, but something I might try first is to check VERY carefully for vacuum leaks. My son's 1984 4.9L [same computer control] had an eratic high idle. I replaced the stepper motor, TPS, MAP sensor, Oxygen sensor and carburetor and to my embarrassment found that I had over looked a vacuum leak.

 

The stepper motor is mainly for emissions isn't it? Slowly bringing the idle back down on deceleration? They still have the mechanical fast idle for the choke? Not sure about A/C high idle though.

 

I believe it does as you described and more. It helps maintain idle speed [up or down], increases idle speed when the A/C compressor is energized and is supposed to increase idle speed when the engine gets too hot.

The carburetor does still have the steps for fast idle.

When experimenting on my son's truck, I decided to take the stepper motor out of the equation. I do not believe the stepper motor feeds back to the computer, but am not sure, so I removed the three [actually 4] screws which hold it and moved the stepper motor to a spot where it could be secured [with wire ties] while remaining attached to the wiring harness.