There's no quicker way to negate your warranty than to show up with a problem and admit you haven't been running your mower with the engine speed control lever full up.
As has been said, this lever does not control your throttle directly, it controls your governed engine speed and the throttle opens and closes automatically to meet the demand. Your mower is designed to run with this lever fully engaged in order to obtain design RPM for engine cooling, hydro cooling, and blade/deck performance. To run with the speed control lever part of the way up tortures your machine. That's why every mower manual tells you to push it to the stop.
If you're OK with the engine and blade noise from your machine without hearing protection, you're not doing it right.