Warmup regulator (engine idle speed drop)
 

Hi.
I have a 911sc 1979 sportomatic and the problem is when I start the engine on cold it runs at high rpm as it should be, but after some time the idle speed decreases only a bit. However, if I press the gas pedal, the idle drops instantly to 950 rpm.
How can I fix that?

Thanks.

It might be a simple fix rather than the WUR.
Take a look at the linkages at the pedal cluster and in engine compartment throttle rod.
Bushings can be worn and binding or in need of some lube. This happened to me, previous owners went and replaced with a rebuilt WUR, I found nylon bushings were worn eventually broke into pieces keeping throttle rpm’s high unless blip the pedal.
2¢

Unfortunately, this didn't fix my problem. I think something wrong with the AAR or WUR. Maybe I need to purchase a CIS pressure gauge kit to do some tests.

As EC900 said, from your description it does not sound like a WUR issue. Something is binding inside the throttle body or along the linkage anywhere between it and the pedal. Next time when it hangs reach down and pull the pedal toward you to see if this helps. Or unhook the linkage at the throttle body by lightly prying the rod off the ball. Start the car let it idle then when it hangs go back and move the throttle plate manually. If the idle drops then you know its not the linkage. If no change then its a spring or bind in the linkage. This way you can maybe narrow down your search a bit.

Remotely possible the plate inside the AAR is hanging a bit as it closes although I have not heard of this happening. Next time instead of pressing the pedal give the AAR a rap with a rubber mallet and see if anything changes. You can pull the metal/rubber hose off and spray lube on the plate inside.

Let us know what you find.

Agree - not likely the WUR. It does settle to the right point - so everything is set as it should be. Something is preventing it getting there on its own. Also doubt the AAR - that won't respond to a throttle blip.
It may even be at your pedal end. Or the linkage binding slightly - there is a kink in the clutch cable that usually sits on the arm that links the throttle system.
Alan

I suspect a stucked plunger in the fuel distributor. How old is the fuel? How often do you drive the car? How long has it been sitting before? At first glance I would top up with fresh fuel or better replace it when it's older than 2 years and fill also an injection cleaner. Then drive the car as much as possible. The plunger should move freely then again. Also the plunger may be risen when the car sits due to vacuum. Some solved the problem with a spring holding the plunger down. You can read some entries here in the forum about this.

Thomas