I rebuilt a 14 hp Kohler and I removed the balance gears. If not install properly and if not with new parts, it can and will explode making a big hole in your engine. Without them, you will notice more vibration, especially at low idle, which seems to quieten down at higher rpms. Some of the lower hp engines didn't have those gears, I know the 14 hp did, and I believe the 12 hp did also. As for rather or not you should leave them in or remove them can cause some debate among garden tractor owners. As I mentioned, if you put them in, be sure they're installed properly and be sure all the parts are in good working order or new parts.