I have a '80 9.8HP tiller mounted on a 14' aluminum boat. At full throttle the boat will do about 5 miles an hour. My transom mounted 55lb trust trolling motor will push the boat through the water faster than the Merc! The motor starts easily and runs smooth. The RPM's increase as I twist the throttle but the boat doesn't gain much speed. I have have checked the plugs, compression and the prop. All are fine. I even ran some Sea Foam through the system to see if it would make any difference but it didn't. I'm at a loss as to what the problem is. I just bought the boat two weeks ago from a guy who had it for three years. He said he used it once two years ago and it ran slow then. The boat sat the remainder of the three years.