Prop/wot trouble.

[Jbegs]

Have a 18 foot 1988 Grumman sportfish with the mercruiser 120, motor runs perfect. I changed out the exhaust/intake manifold as well as the one piece elbow and completely rebuilt the carburetor. I'm running a 15X17 prop and only reaching around 3600 rpm fully trimmed out with me and one other person with no gear and a half a tank of fuel. Is my motor tired or am I doing something wrong

 

[batman99]

Mercruiser 120 at WOT should easly do more than 3,600 RPM. Especially if the engine's sound at WOT sounds even / not struggling.

With key off, put throttle controls in WOT positon and see if carb's plate are in full throttle position.

 

[Silvertip]

Tuned properly????

 

[Sea Rider]

Was OB running better or same before going for the rebuild ? Trim for OB and cav plate to ride parallel to water level once on plane, deck weight must be evenly distributed. If all is ok, could have prop issues, hub on it's way to become spun.

Happy Boating

 

[SkiDad]

What speed are you hitting ?

 

[Scott Danforth]

Is the low rpm something new, or did that show up when you rebuilt the carb?

In thinking you still have a carb issue,

However, do a compression test to determine health of the motor

 

[Jbegs]

I'm hitting around 29 mph and no there is no struggle and it ran exactly the same before I rebuilt the carb. However I had been running the motor at what I can only imagine was an overheat state for quite some time, my manifold and elbow were completely clogged when I pulled them apart. I forgot to mention that when I first bought the boat 3 years ago it would hit around 4200 and reach 32 mph

 

[Jbegs]

I will do compression check tomorrow and relay my results thanks guys

 

[Jbegs]

I have a 14 1/2D X 17P

 

[Jbegs]

Is the low rpm something new, or did that show up when you rebuilt the carb?

In thinking you still have a carb issue,

However, do a compression test to determine health of the motor

My compression was 1)85. 2)120. 3)125. 4)140

 

[harleyman1975]

Should be turning between 5000-6000 rpms. those numbers tell me you need a rebuild. only running on like 2 1/2 cylinders.