I just replaced the idle dampener screw on my 83 300D it was a piece of cake. Just change it while the engine is NOT running. You do not have to remove the oil cannister it will just edge out. you will probably find it is concave instead of convex, if so replace it. Just make sure you get a new o-ring too. When reinstalling it screw it in about 2 turns after you feel it hit the idle buffer spring. then start your engine let it warm up completly, then with engine at idle back out on the screw until the engine start idle starts to drop, then go back in until your idle smooths out. If you have not tried to fix the idle by raising the base idle this should work.
auto\hydraulic mechanic 20 yrs