I'm guessing that a '75 has Bosch K-Jetronic fuel injection rather than D-Jet -- in that case you should check to make sure that the gaskets or o-rings on your oil filler cap and dipstick are in good shape, because K-Jet depends on good sealing of the crankcase to have the proper amount of airflow through the PCV system into the manifold. A bad crankcase vacuum leak, even in a crank seal or rocker cover gasket, can lean out the mixture enough to make the car idle rather badly -- my son's '78 BMW 320i will die instantly if you just loosen the oil filler cap a quarter turn or pull out the dipstick.