My cousin just bought a 2009ish Tracker 175 TX aluminum bass boat which has a 40 hp Mercury 4-stroke. The boat had sat outside for years unused. He got the boat from the original owner who he knew. All the gas was changed. The boat starts good and runs good except he can not seem to get over 4500 RPMs with two people in the boat and a speed of 18 mph. With one person the speed increases to 20 mph. A new 175 TXW with a 40 hp should go about 27-29 mph according to the Tracker website and I am assuming the 40 hp should have an operational range between 5000-6000 RPM's. So what could be slowing down the RPM's? Spark plugs? Tilt and Trim (although I told him to make sure he trims out the motor)?