In a Holley the MJ changes the cruise mixture and also affects the power mixture by how much cruise was changed.
In the Rochester and Carter/Edelbrock carbs, just changing the MJ changes both cruise and full throttle A/F ratio. Normally when tuning these metering rod carbs both the jet and metering rod are changed. Once it is tuned you can swap metering rods for tuning for weayher or altitude. Normally do not have to pull the cover off the top.