Yamaha 150f lost 7mph on top end

[iskeat]

I have a 21ft manatee with a yamaha 150f 1989 (i think) that me and a buddy re-built from the hull up. Our first project boat. We have been running it and wakeboarding with it every weekend since. lately we have noticed that the top speed has dropped and the hole shot has been slower. Pulling someone out of the water on the wakeboard is much much harder. Listening to the motor, it sounds like its not reaching the same rev's that it used to.

Ive check the throttle cable and it is fine. Boat is not water logged. and at idle the motor sounds like its running just fine (no missing cylinders). Any ideas are appreciated.

 

[Yahman]

Some random ideas: Water or crud in fuel? Drain float bowls and check strainer for crud. Collapsed or failed primer bulb? Bulb should pump up and stay firm. Carb restriction? If engine has automatic choke, make sure butterflies are fully open when engine is at operating temp. Fuel line ok? Check at fuel tank nipple to make sure hose isn't swelled (from ethanol) and sucking air. Fuel pump good? Check for cracked diaphram. Spark plugs? Should look clean, dry and light tan, while removed check for uniform compression. Also in rare cases a failing stator can cause weak spark/power issues. That's all I can think of for now, hope this helps.

 

[robert graham]

Check for plugged injector screens causing partial loss of power on one cylinder....

 

[racerone]

Compression test must be done.-----Not sure if there are fuel injectors on a 1989 model 150 HP ???---I think they are carbureted 2 strokes.