Arggg! More 81 sc problems
 

So I hear back from a friend who is doing some upholstery on my new to me 81 sc that the car has developped a poor idle. Where as last week the car made a 1200 mile journey from California to Canada without issue (other then an intermittent no start problem - heat soaked starter) it has now seen the idle drop from a rock solid / steady 900 to a rough 650 rpms.

Nothing has changed. Any ideas where to start or resources ie books to help troubleshoot? Should we check with a vacuum gague for leaks?

other then idle he reports car is still pretty strong throughout the rev-band.

Is the car cold (less than 50 degree F) when it is starting? I wonder if the Warm Up Regulator might be a culprit. In hot climate it might not have mattered so much or even been noticed.

I'm hoping an expert tips their hat in here...

Same thing happened suddenly to me last year. Warm up regulator was the culprit. Vacuum leak could show same symptoms.

what is the elevation change? my 75 car needed a mixture adjust when it gained 3500 feet from cali to texas.

the oxygen sensor relay may be bad, or it's power source has blown a fuse. it's the same fuse as the dome lights. the relay is under the front of the passenger seat, by the tunnel. when you pull off it's plug on a normally good running, warmed up car, the idle drops off as you described. do that test. listen for the idle to fall off.