I think you might have it backward. When the engine is in lean burn, you get less power but more mpg's. Emissions also increase in this mode because the cat is not heated enough to do it's job. When the car leaves lean burn to clean out the cat, then you get the surge of power that you describe.
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-2000 NF Red
-64.6 LMPG
-JVC El Kameleon HU, JVC 12 disc changer, Infinity Kappa 3-ways up front and Alpine 2-ways in the rear
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