Have you measured the actual draw length? Ensured the cam rotation was set properly? How much junk do you have on the string to low it down such as peep, kisser, loop, etc.? With a properly set up and tuned bow at those specs, I would assume no less than 285fps.

The 4" fletching will not make a difference at just a few feet in front of a chrono. Also have you checked the speed through a defferent chrono? Perhaps the chrono needs calibration?