Seeing I will have this off - what are the tricks? I heard something about spraying WD-40 into the valve or adding a restriction inside the hose?
The first thing to do is make sure your thermostat is working properly. My 75 had been completely gutted by some well intentioned stupid person. If that's working all right, focus on the AAV.
Don't take the hose completely off, just take the upper end off the auxiliary air valve and cover the end of the hose. The other air valve is also bolted to the engine and at running temperature it's warm enough to give yourself a nice brand on your palm. The idle should drastically reduce or the engine die completely if your problem is the AAV.
There are a couple of reasons for it to stop working. It has a sleeve inside that's supposed to slide up and down according to the temperature of the coolant water. At just below full running temperature it should be completely shut and not allow any air through. It gets grunged up and that sleeve stops sliding. To do anything about it, you're going to need a new gasket.
Trick #1: Take a rubber mallet, or a hammer with a soft piece of wood and tap on the top of the AAV. It's just press-fit together and if the AAR gets lazy it may not close completely. This is a picture of the AAV on my car and the second ring of the top used to sit slightly higher than the body of the valve. Tapping it in forced it to be closed. On mine, that helped but didn't completely resolve the problem. Which lead me to
Trick #2: Remove it from the engine and clean the crap out of it. Spray it with carburetor cleaner over and over again until the stuff is coming out clean. Then do a thermal shock to force the thing to move. Have a pot of water on the stove at a full boil. Next to it on the kitchen counter have another bowl filled with ice water. Drop it in the boiling water and let it get completely up to temperature then fish it out and drop it in the ice water. Mine took about three cycles to make it slide freely. Try spraying more cleaner into it when it's both cold and hot, to make sure the cleaner gets everywhere. And, truth be told, I didn't use spray cleaner on it. I have a small ultrasonic cleaner and I dumped it in there. That did a lot more effective cleaning job than you'll ever get with spray cleaner. Mine was working so much better after this, but I wanted just a little bit more. So I went to
Trick #3: The lower bulb on this - the part that sits submerged in the coolant when it's installed - is filled with a waxy oil. When it gets warm it expands and pushes a rod that slides the inner sleeve shut. When it gets cold that rod slides back down and opens the sleeve. I don't know if the stuff gets tired or maybe the sleeve gets stuck and some pushes out of the bulb, whatever it is, it stops moving the sleeve all the way up. Use a large pair of pliers, or a bench vise and CAREFULLY squeeze that bulb a little bit to dent the sides. You don't want to crush it, you just want to reduce the inner volume a little bit. That took care of the final bit for me. There is, however
Trick #4: The last resort if nothing else works. I used this as a stop-gap until I got mine working right. I had a piece milled out of brass to act as a plug. In the center of the plug I had the shop drill a 4mm hole. That was my starting point, but the hole was too small. I had a good warm idle, but the cold idle was too low. So I used my drill press and made the hole slightly larger. I did this a couple of times until I got a good warm idle and cold idle.
I had the shop that made mine do two of them, and I'm pretty sure I still have the other brass plug laying around somewhere. I work in aerospace and we have machine shops who custom make things all the time. One of them did it for me as a favor and charged me a whopping $20 for the pieces. If nothing else works, let me know and I'll see if I can dig up one of the ones I had made and you can have it for your car.
Optionally, of course, you could just buy a new AAV. I found one place that sells them for $500.