I have a pair of 225 Salt Water Series Yamaha's on a 1997 Sailfish I bought about 4 years ago. The previous owner had the boat serviced annually at Crocker Marine(closed down). I have put about 350 hours on the motors in 4 years. I do not know whether the low pressure fuel pumps were ever replaced prior to my purchase of the boat. Motors seem to be running well at the present time. How often should the pumps be replaced, as a preventative maintenance measure?