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So what does the upper decel valve do?
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There isn't an upper decel, there is only the decel valve. This is the flying saucer shaped thingie with the vacuum connection. It is supposed to slow the engine's mad dash for idle when you lift off the throttle suddenly to prevent a lot of unburnt gas from hitting the exhaust. Disconnecting the vacuum line from it will often make it shift easier, so people do. I did.
Under high manifold vacuum it allows more air to bypass the throttle to create a more combustible mixture until the engine slows to idle. If you're getting an increased idle when you connect it, the diaphragm inside it is probably leaking. Vacuum leak, in other words, and this could be the source of your problem. Remove it and check; it will difficult to set the idle like this.
ianc
Edit: Rarly was right: the AAR (Auxiliary Air Regulator) should be completely closed when warm. Air through it will still be metered by the sensor plate, so it will cause a high idle if still open. To check, if the engine of fully warm, pinch off a hose to it. If the idle drops, it either doesn't have current, or it needs replacing.