After the car is warm, and I drive to a stop, the car idles at just above 2000. If I rev the engine, it returns back to the high idle. If engage the clutch just a bit to add drag to the engine, I can get the idle back to just under 1000 and if I release the clutch the idle stays there. Once I hit the accelerator, the RPMs move back to 2000.
If I engage the clutch without being in gear, it does not change the idle.

Any ideas???

Vacuum leak. A cracked or lose vacuum hose somewhere would be my guess.

if stock check your `3mm vacuum tee and the lines attached, this has most often times been the offender on my 75.

....& also around the intake boot
(good fortune)

If you hop out of the car when the idle is sticking @ 2000 push the throttle closed by hand on the throttle body and see if it goes back to normal. That is usually the hang up. I put a heavier spring in the bay and it solved my problem

1. vacuum leak
2. check the AAR - when the engine is warm, pull the hose from the AAR to the intake boot Y connector. Plug the Y and check if you have vacuum on the AAR side. If you do, the AAR is bad.

What VALY said.. had this problem last year on my 75, it was the AAR. Pull the hose and plug it with your finger and see if the idle drops.

Same here. After checking meticulously for vacuum leaks and replacing all of the vacuum hoses and intake-related gaskets for good measure, it turned out that the throttle body was not returning to the idle position consistently. My car would intermittently hang at ~1400 RPM when the throttle was released, and would slowly drop back to ~900, where it had been set. A new, slightly stronger return spring solved the problem.

Hope your issue is as easily solved.

-Stephen

Ran across this problem on a low mileage '74 1.8 today. Idles smoothly and consistently at 900 rpm after blipping the throttle while in the driveway, but take it out for a drive and the idle hangs at about 1700 rpm unless the engine RPMs are forced down to about 1000 before pushing the clutch in. I did a complete tune up from the condenser all the way to the plugs and timing with everything in spec. The car had an idle issue before the tune up. The ignition refurbish was the first step.

It's not the TB and I checked the AAR to make sure it was functioning correctly. Now to hunt for a vacuum leak.

Any other thoughts based on previous successful diagnosis and repair would be greatly appreciated!

Isolate and conquer! Remove the aux air valve and decel valve and plug with rubber caps. Then I would look at the air flow meter, see if the air bypass screw is positioned all the way down or halfway up.

I would check under the pedals. Make fire the accelerator cable is not touching the clutch cable.

Exactly. My problem was not recognizing the decel valve. I thought it was the FPR and paid no attention to it. The way the car is acting, it seems like a sticky decel valve.

Good call, but not it in this case. I had replaced the throttle cable a while back and used an inspection camera to make sure that they were not crossed.

Just went through this high idle problem, for me it turned out to be leaks around the base of the throttle body. Solved the problem by taking the throttle body off and sanding the base of the body on a belt sander. Problem solved.

Pretty hard to fix an internal cone with a belt sander ..................

The Cap'n

Capn, just a light sanding leveled the base of the throttle body, high idle is now gone, car runs like a raped ape, so that light sanding seemed to solve the problem.. No more leaks around the base of the throttle body...
BTW the throttle body gasket is new.. didn't have to replace it.

I R&R'd the TB a few months back and noticed that it was barely tightened down. Unfortunately, that wasn't the cause.

Ignition--sticking timing can sometimes do this.

--DD

The culprit was the decel valve which was sticking. Took it out of the loop and no more idle hang. No backfiring on decel either.