Here is another symptom I've noticed. When I replaced the IASA valve, there are three hoses attached to it; one side has a large vacuum hose with plenty of pull; the other large hose pulses. The 3rd hose is a very small vacuum hose. I believe constant vacuum from the small hose is applied to the valve keeping it closed. But when you snap the throttle closed, the momentary drop in vacuum pressure moves the switch in the open position, thereby allowing exhaust gases to temporarily flow back into the left and right intake manifolds; this allows for a reduction of fuel delivery from the carbs to high RPM deceleration, and also reduces the chances of rear wheel lockup. Once you come to an idle or stop, the IASA valve closes and the engine then receives fuel from the carbs.
You're close but off the mark. What that valve does is allow extra air into the intake on high vacuum situations, i.e. deceleration to keep deceleration from being too harsh. If it wasn't closing it just wouldn't idle and if it didn't open you would probably learn to refrain from closing the throttle too quickly.
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