If you have the Bosch KE system rather than the later system as found on the M104 engine, you will find a rectangular shaped, plug in resistor in the wiring harness near the master cylinder. This resistor limits the ignition advance and is put in place as a fixed resistance in lieu of a dial which is placed there in Euro delivered cars. The dial changes the resistance, thus changing the maximum ignition advance. This was put there to allow easy change of the maximum ignition advance so that different fuel grades can be compensated for. Removing the resistor gives maximum advance.

You will need to listen closely though, to ensure that you have no pinging.

Good luck,