The RPMs do not seem right at 34 MPH with a 17" prop. Also, 40 MPH should be attainable. The motor at full throttle should be running around 5,300 RPM, and to get there from 4,100, a difference of 1,200 RPM translates to 6" down in pitch. I would test the tachometer and speedometer for accuracy and then check the prop. What is the make and model? What is the diameter of the prop? It should be around 13". If larger in diameter, it would account for the drop in RPM. Has the prop been reworked? Did you mention the make and model of the prop? Off brand props can be much slower than the Mercury Quicksilver, which are designed by the engineers for the torque and horse power curve of the in-line six. A good bench mark prop would be the basic aluminum Mercury Quicksilver three blade prop in 17" pitch. They are common and a used one might be $75. Something does not seem right currently.