help with an 87 420 sel

It defintely could be the O2 sensor and the need for new plugs and wires. P.

Sounds like multiple problems...

First check all spark plugs for the condition of the electrode... what color is it? Rusty white?

Seconds of all, your fuel mixture is too rich... this can be adjusted by the fuel mixture screw but I wouldn't play with this unless I eliminate other possible causes... such as spark plug wires (check for resistance about 1350 max ohm)... clean distributor cap and rotors contacts... inspect for any damage, if so, replace them both.

Other possible reason is bad O2 sensor... what is the mileage? When is the last time you changed one?

Well I have had the car for 4 years and i have not replaced the O2 sensor. three years ago the engine was rebuilt after my timing chain broke. I do not know if the O2 sensor was replaced. The distributor cap was firing away nicely and did not look dirty. I have not checked the cables other than a look over.

Why not? Isn't it possible that this is the only problem and it's just seriously off? It'd be free and easy to check (3mm allen, stick it down in the hole in the air filter top, engage in bolt, push down against resistance of a spring, turn counterclockwise). Richard

It's stupid to play with the mixture before verifying that the ignition system is functioning properly...

You might cover up a problem, but you won't fix it.

After verifying the ignition system is working right, then using proper gas analyzer or shade tree methods, the mixture can be adjusted.