You can't mow at that setting! The throttle does *NOT* set speed, but rather fuel/power delivered - shaft speed is then directly related to load on the engine. So, what that means is that to avoid overspeeding, you can only set the throttle limit with no load (which is a very, very, low power setting). Hit the grass and speed will either sag massively, or it may stall outright. You would be amazed at the difference in throttle butterfly position between idle and full load with the exact same shaft speed.