lean engines actually produce more overall power. you may have been flooding the engine and not getting a good burn when you were down in the 10s and 11s on your afr. i wouldn't run much lower than 12.5-12.7 to get the best peak power. also if your dipping down then you need to change your cam to one with a longer pump shot.

also if you switched pump your pump maybe overcoming your floats and dumping fuel into the carb. if not then you were set really rich to comp for the lack of fuel flow from the first pump. the carb works on a venturi and if the level of gas in the bowl is higher more will go through the jet and into the engine.