Do you reach max rpm in neutral, like someone asked earlier? This is your most important test right now. If so, then it isn't a throttle problem. It takes surprisingly little growth on the bottom or prop to drastically limit rpm. A couple of barnacles on the blades will do it. If it doesn't reach it in neutral, then make sure the tach is correct, then start looking at the throttle linkage, fuel, etc.
Many engines come with a choice of transmission gear ratios, and I think that one did too. The correct size of the prop will have much to do with which gear ratio you have. This probably explains why you find people with different sized props on the same engine.
To summarize, troubleshooting this problem follows these steps:
1. see if you reach full rpm in neutral. If so, then #2, if not then #3
2. clean the bottom and prop very well
3. check that the tach is correct. If not, correct it. If so, #4
4. all the other possibilities like throttle linkages, fuel delivery, transmission, fuel system internals, etc
I adjust engine power by the quarter wave, because as a displacement boat approaches hull speed the quarter wave grows rapidly. The energy dissipated in the quarter wave is why it takes a lot of power to exceed hull speed. This method also works well when motorsailing.