In my experience in the KOHLER Technical Support Call Center, an improperly adjusted speed control cable is the #1 cause of low power.
Many assume if the engine runs at high the cable is adjusted correctly-this is not true! Max speed requires very little throttle opening (less than 10%!). Max power requires the throttle to open fully, which won't happen if the speed control cable is not adjusted properly.
READ THE SERVICE MANUAL AND FOLLOW ALL WARNINGS!