Editing How to rebuild a Rochester Quadrajet 4MV carburetor (section)
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====Idle discharge ports==== The idle discharge ports that are controlled by the idle mixture screws can be anywhere from around 0.050" to 0.080" diameter. If you find a lean idle condition that doesn't seem to respond to turning the screws CCW, you can gauge the port size using your small numbered drill bit index, then enlarge it one size at a time until you regain control with the screws. Usually about 0.090" is sufficient. This is providing that the throttle plates are not open too far due to a large cam- which will over-expose the transfer slot, just like on a Holley. The cure for that is the same as you'd do with a Holley- start by using more ignition advance. This allows the primary throttle blades to be closed down some to lower the idle speed, which increases due to the advanced timing. Often, this is all that's needed, so be sure to do this FIRST, then enlarge the ports only if still needed. Normally, going larger than 0.0100" isn't needed and if the idle screws are still not responsive at this size, the problem lies elsewhere- like needing idle bypass air (or more bypass air).
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