Even though these are commonly known as an "idle solenoid", they are correctly referred to as an "anti dieseling solenoid" or "idle stop solenoid". Their purpose is not to raise the idle, but to close the carburetor throttle completely when the key is released and prevent engine run on. The reason they came into being is from all of the emmisions equipment, heavy loads like A/C compressors, high output alternators that all require a higher idle speed to stay running. That idle was so high that something was needed to allow the carburetor's butterflies to close completely thereby eliminating engine run on. They were never designed to push the carburetor open on their own. Only to hold it in place until the key was turned off.
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