Pugs and grassman177 are both correct, the stripe flap may need to be raised so it is not as aggressive on the turf and if running flat blades two 1/4" blade spacers will also need to be added to each spindle to properly position the blades. Also check the rear tire pressures, if lowered they may be allowing the rear of the mower to drop when the operator is on the mower (causing flat or negative deck pitch(back lower then front)). It is okay to run lower pressures but the deck will need to have its pitch reset with an operator on the mower to account for the back end dropping. With the operator on the mower the blade tips should have a 1/4" positive pitch (front tips 1/4" lower then the back tips). Also check the engine RPM's (3600rpm +/- 50 RPM at full throttle, no load), it's hard to hear if the RPM's are low but poor cut quality can be caused by low RPM's causing lower blade tip speeds.