SC Idle speed changes with electrical (un)load

The alternator in my 82 is a new one, but the idle speed changes by about 100 rpms when I turn larger electrical items on or off (mainly headlights).

Does anyone know what sort of problem this could indicate? Is the alternator straining the engine under load? Or is the ignition system responding to the change in voltage?

Thanks,

Olivier
82 SC

My 83 does the same thing when I turn on the headlights, probably normal loading.

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Robert Stoll
83 SC
83 944

cars with the idle set low, or real lean at idle, or timing on the retarded side, or all of the above will see more of an idle drop in this situation than one that's more "tuned".