Hello out there!
1990 420 SEl the adjustment allen screw through the top of the airfilter cover in front of the fuel distributor...clockwise or counter clockwise to lean it out. Its running a little rich.

Thanks

You should use the "duty cycle" method. It's been discussed here in great detail many times and should be easy to find with a search.

This is a basic mechanical adjustment that is then modified as necessary by the Lamda system. Most attempts at setting it "by ear" end up a mess.

Duke

Cassandras not withstanding, CCW is leaner. That said, do also check the duty cycle.

Worth buying a cheap duty cycle meter so you can keep an eye on it to make sure it's both correct and consistent.

RayH

Mixture adjustment is the job of the Oxygen Sensor. Has that been replaced as needed?

Which reminds me I need a new one too. Every 30,000 to 50,000 miles so says the following for older cars...

REPLACING OXYGEN SENSORS FOR PREVENTIVE MAINTENANCE

The O2 sensor fine tunes the mixture once the engine's hot. The duty cycle sets a starting point and needs to be adjusted correctly when the engine is hot.

So, invest in the meter. I know I have.

As you say, the sensor should be replaced as a preventive measure. Mine's over 7 years old so a new, MB original one's on the way as I write. I hope to be able to report better town fuel mileage once it's installed and I've set the duty cycle.

RayH