78' merc 1150 no power in water.

[TowerOPower1150]

I have a 1978 merc 1150 on a 18.5 cuddy. I swapped it from a 17' open bow. The motor ran great for 2 years on that boat and stopped running. I swapped it over to the new boat and I had the carbs rebuilt and it ran again, it ran great just like before. I took it out for a test run and everything was fine for about 20 minutes it was running full speed nothing wrong at all. I stopped for about half hour to talk to some friends. Fired it back up and at full throttle I was only moving about 10-15mph compared to the normal 35ish. Motor sounds like it's still running full speed and hitting all cylinders but it's just not moving as fast. What could have possibly happened? Thanks

 

[opalized1]

WITH INFORMATION PROVIDED, I WOULD TAKE A LOOK AT THE PROP, GUESSING ITS KEYED IF THIS KEY SHEARED, THE PROPS NOT SPINNING WITH THE ROTATION OF PROP SHAFT, THIS IS BASED ON SOUNDS THE SAME, RPMS ARE THE SAME, BUT BOAT JUST DONT MOVE THE SAME, NOW IF THIS IS NOT THE CASE, I WOULD WORK INWARD INTO LU AND CHECK ALL COMPONENTS OUT HEADING TO THE SHAFT THAT ENGAGES THE MOTOR FROM LU, GOOD LUCK

 

[jimmbo]

What rpm is the engine running at when it is at WOT?

 

[TowerOPower1150]

I'm not exactly sure the tach on this new boat doesn't work, but by judging off of how it sounded it was running at normal operating rpms, just not moving as fast. It's just weird how it was fine for 20 minutes and after a 30 minute stop it wasn't.

 

[Chris1956]

Mark the brass hub of the prop and the prop blades and run the boat. If the marks no longer match, the prop has a bad rubber hub, and is slipping.