5175 rpm on a Xoar 18x8 is really low for even an 1st gen non broken in engine. I would expect to see at least 6200 and it should be closer to 7000 or more after broken in. I would say either your tach is wrong or you haven't yet leaned it out enough. If you can't get more rpm by tuning then maybe a problem exists with the timing or carb . Verify timing , install new kit in carb, retune.