I've got a 88 Four Winns 261 Liberator with twin 460 Cobras and 22pitch stainless props. Got my GPS out and at 4200 rpms I'm going 49 mph. Played with the tilt/trim a bit but no improvement increasing the mph's. I was told this boat runs 71 mph by the previous owner. This boat weighs 5500 lbs. I was using 91 octane. Any thoughts on what's slowing this boat down?