My motor sometimes reads at higher rpms, than usual. I'm wondering if it's the tach, or is it something else. While at full throtle, it usually reads about 5600-5800 rpms, sometimes it will read 6000-6300 rpms. The engine sounds the same and feels the same. It's a 150 xr6 Merc. A friend suggested, that I might have spun the prop, but wouldn't I notice it at the holeshot? Any help appreciated.