dloveless,

I also have an 84 500 SEL. This is actually a very easy system to adjust once you know how. This is how I adjust mine, and some of the possable problems if it doesn't work.

First, get the engine up to operating temp.
Remove the air cleaner assembly.
Disconnect the o2 sensor.
Get a can of carb cleaner which will in this case act like fuel.
Spray just a very little carb cleaner into the air intake.
If the engine drops in RPM, then the engine is already rich and needs to be leaned down a little. If it speeds up it is lean. Adjust the mixture until there is no change or just a little speed up. At this time, If it is running to fast, then you have an ISC problem, or a vacuum leak. You can find the vacuum leak using the carb cleaner and using very short burst of cleaner, spray around different areas and when you hit it, it will change the speed of the engine. DO NOT spray on the dist, coil, or plug wires. When you are happy with the idle reconnect the O2 sensor.
Now if you have problems starting the engine when cold or after it has been setting, use the carb cleaner to find out if it is not getting enough fuel, or too much. From this you can determine which areas to investigate. If you do have problems, I have found that a pressure guage connected to the control pressure line can tell a great deal about how the system is working.

If you try this, let me know how it goes and we can go from there.

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84 500 SEL (307,xxx miles)