I recently bought a used 2002 Mercury 4-stroke 9.9. When I got it, it was running rough, but would smooth out and run better when partially choked (The previous owner used it as a kicker and claimed to have only a couple hours on it. Don't know if that was true, but it was clean enough to eat off of when I got it.). I rebuilt the carb and it improved the running, but it's still seems like it's short on power. <br /><br />The motor is on a 12' boat and at about 550# total displacement (boat, motor, me, gear, etc) it was running 13mph at 4400 rpms. Prior to rebuilding the carb, it would max out at 11mph at 4000 rpms (and the choke pulled out 2/3 of the way). It has a factory 9x8 prop and it's in good shape. I know someone with the same hull as mine that has a 6hp 2-stroke, and he gets the same top speed.<br /><br />There is no increase in speed from just over 1/2 throttle to full throttle. In fact, it runs best just over half throttle (I'm assuming that since this is the same motor as the 15hp, the fuel flow through the jets has maxed out near half throttle and giving more throttle is only allowing more air and changing the air/fuel mixture.)<br /><br />My dad just bought a 9.8 Tohatsu 2-stroke that pushes his boat (about the same weight as mine) at close to 25 mph. He has a 15" shaft and my boat has a 20" transom, so I can't put it on my boat to compare. So my question is, is there something wrong with this motor, or were these big, heavy 4 strokes just dogs?
to prevent chasing ghosts.<br /> its a method to insure the motor makes power before playing with props but it sounds like your on the right track.