My 1971 1.7l will for the most part idle normally, but will ocassionally drop down to a point so low it barely stays running.
I originally had the idle set at 600rpm. Then, it got cold and it occured to me that I had no fast idle at all. Took the aar/cold idle control valve out and discovered that the valve itself spun freely and when on 12v for 30 minutes (or just heated in the oven) the element didn't seem to cause the valve to close. I removed it from the system by plugging the intake air hose and the hose leading into the throttle body, then reset my idle back to 600 - it had been stuck slightly open. I had originally assumed my idle problem was related to the AAR acting up in some way, but this removed it from the equation.
When the car drops down to the low idle it also seems to be running slightly rich, but normally the mixture seems to be just fine. If I just put my foot into it, it'll stall. I find that I have to coax it up by blipping the throttle slowly. It'll try and idle at this low point for a while then just return to normal after a bit of driving. It does this when the car has been warmed up. It doesn't feel like the car is missing - the idle is low but it feels even enough.
When it's idling low I've tried adjusting the idle screw by the throttle switch, but it seems to have no effect on the low idle. I raised the normal idle to 1500 and found that the low idle was just as low as ever, so it doesn't seem to be related to the normal idle speed.
I had thought it may be a dead spot on the throttle switch, and took it out, but after cleaning it up as best I could the behavior was unchanged. I do have a spare switch, but it's rough.
The only other odd symptom it's exhibiting is a "clatter" above 4,000rpm. I assumed it was a timing issue and have moved the distributor slightly in either direction with no effect. Not sure if it's related, but thought I'd mention it just in case.
Alternator is new, so I don't initially suspect an electrical cause.
Fuel pump and fuel injectors are new, fuel pressure is set to spec and stable. Valves have been adjusted recently. Points, trigger points, condenser, cap, rotor button, wires and plugs are all new. I have a weak set of rings on #1 but otherwise no oddities.
What might it be?
Thanks!