68 40hp johnson power and speed

[invader_1230]

i have a 10' GW invader, and i have a 1968 40hp johnson on it, and it goes 47 km/h. which isnt too bad, but last year i was running a 1955 25 johnson and got 46km/h, so there must be somthing wrong. and i was reading about someone with the same boat and a 50hp, and the boat goes 90 km/h. it doesnt slow as much when carving with a slalom ski, but i still feel like i should be getting alot more top end beacause it gets to a plane at about the same speed as the 25. the prop says 10 3/8, so maybe my prop is totally wrong. the engine idles and runs smoothly, but i just dont have much speed, or a ton of power. and when i am going full speed it sounds like its reving out about where it should be. so is this known as a slow outboard, or the 25 as a fast one. also i cant find the high speed adjustment on the carburetor.