The air mixture screw does not make my mixture richer or leaner

Hi Alan:
A bit more info would help, like what kind of vehicle, and any symptoms.
Check the inside of the exhaust pipe. sooty is rich, dry is leaner.
Check the spark plugs. sooty is rich, dry is leaner.
Check the mixture. screw in for leaner. If RPM increases, it was rich. If RPM decreases, it is already lean.

When a car warms up it enters closed loop state. This is when the computer uses the sensor inputs to adjust air/fuel ratios.
MAF - mass air flow sensor - These can be cleaned.
Oxygen sensor - Usually when they fail the mixture gets richer. I think yours is getting leaner
Camshaft and crankshaft position sensors - They can be affected by heat when one is failing. They provide the computer with timing information

An obd tool that reads real time data would help narrow down the problem. You can get an elm327 usb to obd connector on ebay for $10. Then get free software to read the car computer with your computer.

turn allen 3/32 allen mixture screw for each barrel counterclock wise for leaner fuel mixture if turn mixture screw clockwise richen fuel mixture.

usually turning screw clockwise sends the main jet down (opening more) getting more fuel through (richer) and opposite is 'usually' for leaner, or 'weaker' mixture...
get hold of a Colourtune carb tuning tool.. this will help endlessly..

If you ignore this it could damage the catalytic converter and that would be exspensive.

Your going to need a professional that can
figure out how to restore your very old vehicle
to OEM Specs,so you can pass inspection

Fords are notorious for Idle Air Control solenoids (IAC) going bad and sticking - especially in cold temperatures. They almost never set fault codes in the computer. I would say that there is about an 80% chance that if you replace the IAC your problem will go away.

The Automatic Choke is not working correctly. If you are flooring the pedal, and I am assuming that you are running a Computer operated ignition system. So, then by opening the throtle, you are just letting more air into the motor, and leaning out your gas to air ratio, which is controlled by your computer. If it idles Ok as soon as you get it started, then I would say that this Choke issue is based on a faulty (MAF) sensor, and since the fuel going into your engine is controlled by your computer and the interaction it has between the throttle position sensor, and the Mass Air Flow (MAF) sensor, to regulate how much gas to inject into the throttle body at any particular moment.

If this is a carberator engine, then the choke issue is easy to fix. On the side of the throttle body (passenger side) is a round bowl looking thing with a plastic cover, held into place by a couple of tabs and two or three screws. If you look close enough on the plastic cover you will see teo arrows poiting in opposite directions, one saying leaner and the other saying richer. Loosen these screws just enough to be able to turn the plastic disc cover towards leaner, about a quarter turn to half turn, and tighten up the screws again. This should work...

they adjust the air intake and fuel intake witch is the rich lean and you should adust them all the way in and adjust them out as needed you will have to play with it. and guess what its an 81 it is a cold blooded beast please keep it on the road love the classics

1988 should be fuel injected. The ECM controls the fuel mixture and it isn't adjustable. What kind of problem do you have? If it is 1 ton or bigger truck it might have a carburetor. If it does, let me know, I will tell you how to adjust it.