The pistons are raised by the vaccum created by the engine, not the airflow - the greater the vac relative to the airflow quantity, the higher they're raised hence the weaker the mixture. Putting stronger springs in or stretching the original ones will keep the pistons lower thus increasing the air speed over the jet, increasing the depression and making a richer running mixture. It has little or no effect on acceleration enrichment. Spring stretching has been a quick fix for lean running with Stromberg CD series carbs for a long time because nobody dares move the metering needle in the piston for some strange reason. Not a good fix but cheap and quick. (many Volvo dealer mechanics used to do it!)