need help from someone with old merc experance

[lathehand]

I have a 1977 150 hp merc1500 that won't rev up over 2500rpm.I've tried everything from rebulding the fuel pump and carbs changed all fuel lines and water seperator .Done new plugs wires and cap rechecked the link and sync and still ng.I'm not very good with timeing and carb sync.Out of gear it'll scream and it will run all day at 2500 rpm.One more thing I checked the compression and found all 6 cylinders are within 12 lbs of each other.What have I overlooked