Timing 34* total. It may be a jetting issue...but as stated before, if your turning a 21p at 5100....anything bigger is gonna pull the rpms down into the 4500-4700 range. I think your down on power sir. My old boat (25' Chris Craft Limited) had a slightly warmed up 502 in it. I had a labbed 25p Mirage Plus on it...and it went 67 mph (GPS) at 5000 rpm's. Do you know what jets are in it now ? Did you look down the top of the carb to see if the throttle blades were perfectly vertical with the throttle pushed all the way forward (engine off) ? This will tell you if your throttle is adjusted right, and your actually getting WOT. Just an idea.