"Finished" boat rebuild this week, discovered new problem on lake

[RonWilder]

I just "completed" a total rebuild of an '83 bomber I/O. Replaced a seized 470 with a 153 cid. I put it on the lake and all went well the first time out. The second time however I noticed a loud knock and vibration when trying to idle. Took it home and tightened the motor mounts on the transom plate as much as possible, and checked humble bearing and universals. Boat runs flawless at high RPMs and has no knock/vibration. When idling however it knocks and kills the engine. My boat mechanic said he has no idea what could cause it.

 

[jbcurt00]

153, Mercruiser 2.5L, correct.

Moved to I/O forum

 

[RonWilder]

Fixed

 

[Alumarine]

Fixed

Awesome. Mind letting people know what it was?

 

[RonWilder]

My gear ratio is set for a 180 hp, the new engine is only 120 hp. Normal idle was bogging down and killing the engine in the roughest manner possible. Made my idle stronger and now it has no problems

 

[Rick Stephens]

That's not fixed, that is moving the issue to a different day. You need to prop down a bunch so the engine is not loaded more than it can handle.

 

[Fishermark]

^^^ Yep. Definitely need to change props or you will be changing engines again.