Evening guys.
Problem vehicle is a 1987 260E. 98,000 miles believed genuine (yeah right!)
I can put new clean spark plugs in the motor and within 3 mins of idling they are covered in black soot and the motor missfires eventually stalling.
Running rich is my guess.
I've read every thread here (and there's a couple off hundred) with regards to possible reasons. My question is before I start throwing parts at it... what's your best guess'

I will take the most popular guess and run with it

Michael