I replaced 2 fractured vacuum hoses - one (white) that runs to the warm-up compensator from the green side of the white-green valve, the other (white/blue/violet) from the air pump to the advance unit. No change.

Replaced the OVP relay. all 8 plugs gapped at the recommended 0.8mm (Bosch W8DCO copper, no resistor), checked resistance of all plug wires from boot to cap (all 5kohm), replaced distributor cap and rotor (overdue anyway) and nothing changed.

Checked resistance between terminals 1 and 15 of the coil - came in at 0.2 ohm, slightly below factory recommended 0.3 ohm minimum.

Followed the diagnosis in the MB 116.96 factory engine manual at 15.7 III-562/7. This states that pin 3 of the diagnosis socket (LH front wheel well) should return battery voltage against ground with the ignition switched on (I asume that means engine not running). I got only 3.80 volts. Next step is to test voltage between diagnosis socket pins 5 and 4 which should return 0 volts - I got 2.81 volts. Book says this means the ignition switching unit s/b replaced. Before I replace this expensive part, does any one have any input which might invalidate my diagnosis?

Once again, the engine runs rough at all speeds and temperatures.