Editing How to rebuild a Rochester Quadrajet 4MV carburetor (section)
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===Power piston=== From quadrajetparts.com: <blockquote>'''Testing a power piston'''<br> You can easily test your Rochester Quadrajet power valve spring to see if it is too strong. At idle, the engine vacuum needs to keep the power valve fully seated (the fully down position is the leanest position). Take a normal plastic drinking straw and place it in the top vent of the airhorn, by leaning the straw on a slant you should be able to feel the power piston as you press the piston down and up. With the engine shut off, place a mark with a marker on the straw showing to top height of the vent. When you start the engine, the straw should pull down and stay down. If you see the straw move up and down, you know that the power piston valve spring is too strong for the vacuum produced by the engine at idle and needs to be replaced with a lighter (weaker) spring. Also if the power piston is moving up and down at idle, the engine rpm will also be unstable and will rise and fall.</blockquote> This will also tell you if the power piston is stuck in the up position if you suspect it might be, like after a rebuild.
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