AFB starves on the secondary side

Is he bogging because he is too rich? Lighter spring would keep him in the primary longer and get him into the power enrichment later, maybe past that flat spot?. May help him over that condition. Stronger would send him right to the power enrichment with the absence of the required airflow. If this has just started, the springs could have fatigued and become 'lighter' and are now preventing the power enrichment at all. Send for a A/F meter!