1992 mercury 100 no power

[gonefishin72]

ok i'll explain this the best way i can. Just bought a 1992 17 1/2 ft spectrum with a mercury 100hp outboard. everything is original one owner boat. he says it would run out at about 40 mph. i took the boat to the nearest lake yesterday to work any bugs out. (boat has sat in the garage for a year) i put a new water pump impeller in it and tested it with muffs first. got that working. engine runs flawlessly so it seems without a load on it. in the water when i give it WOT it stand the boat on end. real trouble planing. rpms around 4000 i think. (no tach) definately not where its supposed to be.only could get it to 30 mph. so i checked the prop to see if it was the right size. it all checks out 13 1/4 X 17. does anyone have an idea why the engine is not revving up? also it porpoises real bad if i try to trim up any. even when i trim up engine rpm will not come up. makes me think this is an engine prob? carb issues? im kinda lost and am heading out on a fishing trip in 3 days. any suggestions would be appreciated!